I owned a C150 with an auto gas STC. It really wasn't enough horsepower for going anywhere except in circles especially in the Rocky Mountains. A&Ps have told me to steer clear of an aircraft burning autogas because it causes more problems than its worth. If you use the STC absolutely make sure the gas has zero alcohol additive!!

The Cessna 152 has a little more horsepower but I would go with the Sparrowhawk with a 125 HP. If you live close to sea level the C150 should be fine. It's a rock solid airplane that will hold its value! Pre inspection purchase need to to check the firewall isn't bent and there is no corrosion. Look for a low to mid time engine. It's a trainer and folks don't keep the nose gear off the ground in the flare. Remember to make two landings, main gear then nose gear.

I think its a great idea to time build.