Re: Which kit?
I've never read of any actual proof that Grubees are any better than other motors. The only advantage seems to be that you know better what you're getting into, and compatible replacement parts are widely available. Other than that, a Grubee is not necessarily any better than any other HT bike engine. I've never owned a Grubee, so I may be wrong.
To be honest, I'd suggest not even buying a kit and instead buying the motor by itself and all the parts separately. That way you can start out with quality parts that will last you. Most of the parts that come with the kits will work for a few weeks, but in the long run will cause you problems and need to be replaced. Buy better quality parts & mounting hardware, do a teardown of the motor to polish/clean/fix it, mount everything properly, and you'll have yourself a bike that you'll actually have fun riding instead of having to constantly fix things.
There's nothing wrong with getting a kit if you're a first-timer, but be prepared to replace the plug wire/fuel line/fuel valve/clutch cable/spark plug/mounting hardware/air filter/idle screw/sprocket bolts/chain tensioner.