Restorers only modify your Soft Defense (SD) which is capped at 900. Since the SD cap is very easy to reach, running a restorer is pointless.

The crew you need to groom, is the crew you will be using in the SS4 and SS5.  The end-state crew for the "SS6" is unknown, but will probably be 1 more support slot than the SS5 (at 5 slots). It could be another gunner (like SS4) too.

Anyway, a couple of notes about subs in general will help direct your crew development.

1) SS1 is simply obsolete.  It had difficulty with SS2, but the SS3 simply puts it away.  Too slow, not enough air and the competition's sonar picks you up a mile out and sends torps before you even see them. However, it has a nifty mine-laying setup in the rear.
[OPTIMUM CREW: BO, SON, PLN, 2X Torp, 1 Gunner or 1 Engr]
Learn to use the SS1 and move your crew between ASW work and SS work. It is good practice.

2) The SS2 is also obsolete. Not so much that it can't sink a ship or two, but as with the SS1, its sonar is too weak to pick up higher level subs' sonar and you're dead before you know it.  However, you can still sink pretty much any other sub in the game if you can hit them with your torps.  The KM SS2 is essentially the same layout as the KM SS3,4, and 5 with 4 front torps.  Only the SS5 has 2 rear torps tubes.  The SS3/4 get better torps. The SS5 gets better torps.
[OPTIMUM CREW: BO, SON, PLN, Engr, 2XTorp, 1 Gun in R slot]

3) The SS3 is essentially the same layout as SS2, just a better sub. Same crew limit.

4) The SS4 is where you will want your two huge gunners ready to go.  The U-Flak is one of the best AA ships in the game, and it's one of the funnest to play.  It can run full speed at 40 OH, with a decent crew.  It could very well be the 2nd best sub in the entire game.
[OPTIMUM CREW: BO, CHF PLN, CHF SON, CHF Engr 2XHvyTRP, 2XHgeGnrs in R slots]

NOTE: If may need to cycle your Gunners throughout their development to get two of them up to level for SS4. (Or just go to NF Trade and buy one).

5) The SS5 loses a gunner slot and those wonderful U-flak guns and replaces them with  the useless 4" potato gun.  That puts two more support slots upstairs.  Now you need 3 engineers, and you've only been training 1 of them. (Back to NF Trade).

[Optimum Crew: BO, Chf Son, Chf Pln, 2XHvy Torps, 3X Chf Engr, 1 Seaman/Repairman or 1 Gunner in R slot]

Just an opinion piece here:
I like to use my deck guns.  I have sank many ships and shot down many planes with the U-Flak guns.   I have also sank ships with my 4" guns.  The 4" gun is good for sinking that CV that just needs to be pushed over the edge without wasting a valuable torp.

The U-Flak guns can rip a DD in half in seconds or destroy a squadron of T4s camping the BBs. 

As for using a repairman or Seaman or Restorer in that R slot for SS5...  I use the Chf Seaman because it pushes my SD to 900 without a rep/rest and increases my Sonar, Planesman and my engineers.  Right now my sub pulls 40 OH.  However, without my Seaman it slows to 39.  No big deal, but everybit helps and in SS vs SS duels, that tiny fraction of sonar pickup or extra air recharge, or torp reload makes a huge difference in success.  With subs, the repair and restore values are amplified on support slots, so even your sonar and planesmen contribute to your SD.  Adding a Seaman increases those values, increasing SD overall.

Also, I run full crews. Max rookies/experts/vets all the time.  My sight and range pickup are a lot further than other subs.  I also add armor.  I will apply maximum armor without speed degradation.  Not a lot, but a little bulge, bulkhead and belt can make that huge difference in battle outcome. 

So, think of the crew you will be using in the end, not the crew you need to run the sub you're currently using.

I'm keeping my Gunners for SS6.

@ULtra_Dog: u said KM SS4 is the 2nd best sub in the game. which one u think the number one sub?

I am going to have to disagree with ultra_dog on principle.

#1  He has always run 40knts.  You may think oh what is 1 knot.  It's the difference between turning faster that the other SS in a duel.  It's the difference between being able to stay within torp range when you are chasing after a BB or CV for the kill shot not just exp.  It's the difference between being able to grab some air out of range inside enemy territory. 

#2  In my opinion a Seaman is a complete waste of a slot.  Jack of all trades master of none theory there.

#3  You are not going to be able to afford ANYTHING worth while in the Trade Room.

If you are building a crew you need good planesman and good engy(s) or ur no threat.

I decided to develope a KM SS line to do SS fighting. The prox torps are the best sub fighting ordinance. Period. IMHO, the US GATO is the best overall SS. (and will reinforce that during game play), but the KM sub(s) with prox torps make them, hands down, the best sub on sub fighters. I have tried to prove it different, but as hard as it may be to accept, this is the truth.

For the practical input on the KM thing tho, it seems that the KM SS4 setup is best realized with;

BO, sonar, and planesman... no surprise, these guys are needed for any SS crew. I run my BO as lean as possible, with the understanding that vets/experts on him helps the SD early on. As growth in crew increases, I lean out the BO as much as possible and after that, the sonar man. The downside on a lean sonar man is that initial contact position with other SS's can be compromised.

x2 torpers. Goes without saying that you will need the torper(s). My front torper gets the same expert/vet attention that my eng's get. Spend everything I can afford to keep the % as far above 100 as I can. Higher as the great gods of rolling sailors allow. While I lean out the rear torper, he's indispensible in a sub battle. Leaning him out is gonna reduce your reload time, so do accordingly.  

x2 eng. And I spend everything I can on experts/vets. (I dont have a KM SS5, but the extra slot would go to an eng if I planned on making this my main SS line.)

(x1 repairer - for the SS5). While a restorer gives high SD early on, I'll take the hit early on to develope a repairer to make that 900 SD mark a little later. Work on his expert/vet % to be above 100 as high as possible.

  x1 gunner, in the rear gun slot.   The rear slot x2 guns are active with a single gunner. For me, the high speed guns are the funnest thing to come along since sliced bread. The distraction of aiming the gun(s) since I'm on the surface (and very vulnerable) only makes it worthwhile when catching a crippled, non-HH carrying CV or a stinking FF/DD that I can cut in half with the lead hail. Along the lines of winning the lottery, but it NEVER gets old when it works. Would never give up the forward gunner slot, where I carry an Eng, for the single added gun. If gave up the gun I'd replace this slot with a repairer.

As a note... IMHO... forget using a seaman. Our tests have shown a seaman to have negligible advantage on a sub. Negligible to mean... worthless. On several nations SS's.

Having said all that, I'm not all that good a sub driver. Just sorta dangerous on a good day. And all my thought's didn't cost you a thing, so I suppose that in the final analysis, that is exactly what they are worth.

Good luck in building your crew.

Peace.

 ETA: I spend my entire available armor on buldge. Maybe 1 unit on belt armor for theose first BB round hits, but since I'm primarily focused on sub fighting with this guy, again IMHO, bulge is where it's at. 

Carry on.