Welcome to the forum.
Sorry you haven't seen any responses to your questions but they are a lot like asking what's better, Ford or Chevy. You'll get very different opinions as to where to buy the "best" engines/kits. All will be based on the experience folks have had. You'll find most 2 stroke bicycle engine kits to be nearly identical. What we stress is to buy from a reputable dealer who will stand behind his sale. Some of the cheaper ebay sellers are only drop shippers and care less about any problems you might have after the sale is done.
We have a very comprehensive 'Search' feature that can provide you with answers to most of your questions. But, there is always someone here to offer advice. As for the 'jackshaft', or, 'shift kit' issue there are those who use and like them and there are those who have no use for them. It will come down to personal preference on your part. Lots of reading on that subject. Try the 'search', type in 'shift kit' and you'll have plenty to read.
Good luck and have fun.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"