Give this link a hit. NADA guides aren't gospel but can give you some overall guidance based on condition, geographical area, and brand. And they will be what a bank looks at for loan values. My one rule of thumb with any boat purchase "ride before you buy". Simulate how you will use the boat; planing off, idling, lowest planing speed, mid range, top end, fishing, etc. If this boat with a 90 will truly GPS 50 that's scootin'. These boats were packaged with as small as 40 hp engines but I can tell you that wasn't an ideal set up.
Used Boat Prices ? NADAguides Boat values, Marine prices and Outboard Motor prices