I will try to give a fair answer here.... first a Rivermaster is a good running hull. Only inspection will tell if this particular hull is in good shape. A healthy 305 will run OK in deeper water, the issue is dry running. Airboating is a disease.... that makes one want more and more power.... you may be happy to be running in the swamp but you will feel limited.
The question you face is buy now.... and then put more money into making it the boat you want.... a long and a xpensive processor or wait and buy more boat?
A friend had a 350 DD on a Rivermaster... it was fun.... but we got stuck a lot ... it was good on gas ... ran deep water OK but forget about running dry.
If you can live with that then it might be the boat for you ...
Good Luck
AirHog