Welcome to the forum. You've found the best place for help and answers to your questions.
Unfortunately the days of under $100 kits are probaly gone. They've been steadily increasing in price over the last year or so. There are a few still available on ebay for just over a hundred but beware that there are some disreputable sellers out there who will not stand behind their products. Our advice is always to look for a good seller instead of looking at the bottom line.
As for mounting on a stretch, here's where to start > Stretch Cruiser & Chopper Motorized Bicycles - Motorized Bicycle: Engine Kit Forum
Good luck, have fun.