When you first asked ... I pulled up the WS website to see what approach I might start with .....
1. Leave in boat speakers as is
2. Do a pair of Rev10s off a dx4
3. 12" Wetsounds (for matching purposes) or JL w6
4. dx6 to power the in boats and sub.
With a 5k budget, that leaves room for box building and tuning from a reputable local shop. (assuming you are doing the rest yourself)
There are many other considerations (future plans, upgrades, downgrades, etc) that may affect amp choices but that is a place to start conversation. It is also a very solid stereo system.
A factory dual battery setup would likely be adequate.