For $150 you're only going to get a basic 2 stroke kit. The good news is there's many on here who can tell you how to tweak the engine to get its max performance. Then you can buy upgrades from the sponsors as you can afford to do so.
if you want a good kit at $150, check out the single piece head/jug kits on amazon. amazing power!!! i swear by them. guys spend gob's of money buying parts these ugly little motors put on from the box. so far the one si have are all better then even the pk-80 kits. more torque, and more rpm's right from the get go. an after break in, wow!!! with my 36th sprocket i still run up hills fast and hit speeds far faster then i feel safe riding at. i keep a 40th sprocket on now because i get froggy and need to keep my self under 40. lol. with the 44 supplied in the kit, i was popping wheelies and spinning the tires. i could climb tree's with a 44. with my 40 tooth i hit a top speed of 37mph. i prefer to stay at 25. out of the box every single one of my single piece jug/head combo's were pulling 8500 to 9000 rpm's!! best part is no head gasket to blow, bad thing is not as customizable . but i don't care with the power they make. an ik guys spend gob's on parts for other engines, i was and kinda still am one of those guys. i have probably 6 motors laying around at different stages in build. i don't sell these, i play with them. i'm a motor addict and this hobby is one i can afford. i have about $9,000 into this hobby in the last idk how many years. i've killed a few and made most sing. spending money doesn;t make these go faster, knowing the gears and tuning them right and the proper mod's done by hand , does. a dremel will be your friend in this new adventure of yours. just remember, baby steps when porting and matching. baby steps so you don't ruin the jug.
Much depends on what type of riding you most prefer, & your health & condition. Do you plan
to use pedal assist most of the time or throttle? How much riding & mechanical experience have you?
In any event, I'd save up a few bucks, wait & get what you really want cuz $150 don't go as far as
you'd like it.
Apart from that, a basic is a sturdy 26" preferably with 'decent' front shock works with most kits.
Some prefer a different size, but generally speaking, 26" tires are more available & cheaper.
I have had good luck with all types of engine kits, from grubee to bottom dollar eBay. Both that I’m currently running are from eBay, the oldest is a 2011 48cc straight plug head design. After a couple of modifications it runs well, 30-31 mph with a 44 tooth sprocket. The 66cc is a 2014 eBay, it also has been tweaked a bit and will do 38-39, I don’t really know because I quite before it did. If you’ve ever been that fast on a contraption that was designed to go 10 mph you know why.
Yup, I know why. My preferred cruising speed is 14 mph. I ain't in no dam hurry. My current ride is
set up for touring, with or without the motor. It mushes along at 14 mph a mite above idle at 3200 rpm.
It's a slightly modified GEBE kit with a 40cc Tanaka.