Assuming you are talking about KM then yes, the AD is quite overpowered so you should be good with that one.

The Moltke has very good AA and it is also worth buying, you might want to use it for AA in GB and earn some experts.

As tapp said, is you are going to KM line, AD is highly recommended. KM ships have a little less support slots, and as AD is in the middle road to BB6 will be a good leveling platform.

Molkte eis good as AA platform, and can give some help on warfleets because the firepower same as some BB1.

Bayern, sure NO to buy it. 

I have AD, Molte and Bayen. and agree with most of the above. However Bayen in Blitz with 3X6 and higher level gunners is a nice ship and can hold its own.