If yer gonna stay with a single wheel, non-swivelling wheel, you could go with the back end of a scooter as part of yer initial build. Just rip the motor and transmission off and you have the suspension and wheel setup to weld the rest of the connecting pieces!
Here's a scooter fer $50!
https://houston.craigslist.org/mcy/d...476136536.html
Heck...you could even use the back end of a kid's bicycle as yer initial build as a test.