I'm no expert - so others will chime in:

A proven kit that others have put together with no major issues. Knowing someone who has done that kit is good, but asking on a website is fine too

80cc engine with good bearings and non-cheesy carb
Good clutch
Good gas tank with real petcock
HD mounts and frame adapter
Real clutch handle and nice twist throttle
HD MC chain (not a bike chain)
Good quality sprockets and chain tensioner
Good real English instructions