Cruiser 26 single speed, me 280 lbs I’m looking for most simple reliable mid drive conversion to ride 30 to 40 miles max speed maybe 20 mph

ok if your use case is mostly flat easy terrain and commuting, id suggest a bafang bbs02 750w. theres also a tongsheng 450w torque motor, idk about it, look up some reviews. bafang is pretty solid and smooth. for 30-40 miles of range on a charge with efficient riding youll want a 52v 17.5-20Ah pack. if you need serious climbing power with your weight and the bike, a bbshd 1000w might suit you better, has plenty of power.