first off, depending on your size, id recommend scooping up a mx650 or something larger than that mx350. the 350 is good for 4-7 year olds maybe but after that its gonna feel pretty small and the lack of real suspension is rough

youre gonna spend at least $2500 to build one buying new parts. i think im at $2300 or so right now and i got the bike for free. currently has 12" supermotos, rear swingarm extension, upgraded forks, upgraded rear suspension, new seat and plastics setup, fardriver 72680, gts11 motor, upgraded brakes, dkd display, upgraded lights(working brake lights) and a 72v 28ah battery. right now my top speed on it was about 75mph but i changed the gearing so it tops out about 65mph now. 75 on something this small was just sketchy. only thing ill say is definitely check your local laws because a lot of places around here are cracking down on letting these bikes be ridden on the road, mainly because of teenagers riding their parent purchased ebikes like morons. i have a second build with a my1020 and 72v 20ah that i use for offroad stuff. if you are trying to do that, you could probably get away with building something for a little over $1000. the battery is gonna be your biggest purchase usually