I have one of these 31 Decathlons and really like the boat. Lots of room to move around or take others along for a ride. The cuddy (or coffin box) is enough to overnight for 2 if your expectations are not to high. We have managed for a few days in a row.
If you need more room think about having some canvas work done that can attach to the arch/bimini top to the front and back of the boat. I seen a couple of Scarab Sports set up so that you could stand up inside when docked at night or to get out of the weather. Their is a pretty big area in the front between the cabin and seat that you can fit a big air matress.
It was shown as being Completly redone by Cigarette in 1999 and the boat looks good.
I would think speed would be around 60 mph with that power. The boat is heavy with mine weighing in around 7700 with no gas but has another motor off the back. It handles Lake Erie within reason and always feels stable.
There were between 20 and 30 of these boats produced and then they went to covering the deck and making the 31 Bullet.