hi there, my advice is that you would need at least a 5.5 the ideal engine is a four stroke as they use alot less fuel!!!! you will need aboute a 135hp 4 stroke for 6 divers in a force 3+ wind to get through swell etc with no fuss.
Humber have high tubes and a good beam, however they are not the cheapest. Tornado do a great rib and make a good dive boat. have the console near to the front to have good use of beam for kitting up. You should have a good high transom as a low one will let water in when divers kit up near rear when there is a swell. Also a high nose is good for ruff water should you be caught out. On a 5.5-6.5 mt boat a 5hp aux engine is great and works well.
If you are getting a secound hand boat always put to water in land or sea not just runnning on muffs as it will only show major probleam when you set it to go!
Check piston pressure in a two stroke, make sure water is warm from tell tail. Check all engine warning alarms (if there is any) work in case these have been de-activated to hide a oil over heating issue or such like. If it has an internal hull for fuel and this is not being used ask why! could be a real issue, and slab tanks take up alot of space when you have a full boat.
if you fit a bottel rack make sure the boat is weighted for it! as it can bring transum down and then it will let water in.
All of the above we have found out over the last nine years!!!! good luck