re: which is the best kit?
I am also new here, but let me answer your question with another question..
What kind of budget are you working with?
If you don't really have one, I'd say go with a Morini motor. Now, this isn't a kit, so I am thinking it would be out of the question. But it has been said to be the best and most reliable motors.
Now, to the part you probably care about the most.. All of the kits are actually quite similar as far as I can tell. There may be slight differences, but in the end the ones you will most likely look into getting will be all Chinese manufactured.
I personally have a Grubee Skyhawk gt5. It has been running quite well for me and I have only had to do a few minor tweaks to keep it going strong. After about 2-3 weeks of getting to know the engine and getting it working properly, I feel it is now running with no problems at all and I can fully enjoy each of my rides.
So, where do you go from here?
Well, you can wait for some more experienced members to chime in (which I would suggest) But while you wait, do some research on different kits that you have your eye on. Check the options out on different sites.
Btw, I am thinking about making a switch to the friction drive 4 stroke eventually. You can install those kits in an hour or less. They seem more reliable, but 4-strokes run at lower rpms, so you may be giving up a little bit of high end speed. But the thing will still be pretty quick! 4-stroke kits are a little more pricey too, but for sure worth checking out.