The sooz 140 is the most common you'll probably find on there. I think they are rated to 150hp though, check the ribcraft website out for specs.
Personally I'd want 115hp minimum on there and preferably the 140. As with anything used a compromise may need to be made. If you like the grey one with the Honda then go sea trial it and see if it suits your needs, you may be happy with the 90 as you have never had the bigger ones. If the hull and tubes are good you can repower at a later date if you feel the need.
Here is a video of the boat I assume your looking at https://youtu.be/p0ZtxT1GMUA
the tubes look bare in places on the pics and video, needs a closer look.
Where to look is to keep googling really as they pop up in weird places.
BEWARE of scams, plenty about with this boat as it is so popular. You may find some very young 585's for less than 10k, total scams. Do not pay ANYTHING unless you are standing in front of it with the seller, not even a deposit.
Expect to pay 14k plus for a decent one circa 2006 ish and over 20 for more modern examples. This is assuming Max Outboards on there, be less with fewer ponies obviously or with high hour motors.