Here are pics of the insides of a Rotex tank. I got the demo tank which has a big hole cut out for trade shows but the innards were good.
This tank has a large HX which is big enough to do DHW in a conserving household, a coil for the solar which is at the bottom and one for a backup boiler, plus maybe one more. It was to go into a tank like yours but I messed the tank up so it needs some adapting. ....pick to come