Those are very good prices for dacron. My boat is a little bigger but was quoted around 1200+ for a mainsail. The main I ended up getting was around $2200.
If measuring worries you, might want to consider hiring a sailmaker to measure your boat. Although, that may be frowned upon, most of the time sailmakers expect to make a sale after doing the leg work.
The 7.5 will be fine. I have a 3000 lbs boat with a 3.5hp motor that moves just fine. Although, when the wind and waves are up, it's just better to sail.
Merit 25 # 764 "Audrey"