98 up it or not

The M16 kit is useless. You end up with a marker that looks ridiculously NOT like an M16, but enough vague similarities to scream 'POSER!'. There are far batter barrels to be had. Flatline would be reccommended, but if you're allergic to FLs, a Lapco Bigshot or a J&J ceramic would be in order. If you must have a milsim, pick up a Sim-4 or a Sim-5 from Ariakon. The Sim-4 doesn't feel at all like an M16, but anyone who hasn't shot the weapon won't see a huge difference.

Response trigger is pretty good, but not always tournament legal. I don't know enough about hoppers, so I'll let someone else deal with that.

As for a stock, I reccommend the Carbine stock. Read my review on it at http://www.pbreview.com/products/reviews/?sort=new&prod= 2051&page=1#116730 . As for a sight rail, any decent sight rail will do, but with the stock you'll need a pretty high sight profile. LApco sells decent sight rails for the Tippys.

The order of upgrades depends on whether you play woodsball or speedball.

For woodsball:

Better barrel, stock, remote line (not in your list, but vital for woods), electronic hopper, then whatever the hell else you want.

For speedball;

Better barrel, ditch the RT entirely and get an electronics kit for legal tourney play, drop forward, double trigger, and then consider getting a constant air source like high pressure air or nitrogen. If you stick with CO2, get an expansion chamber or a Palmer Stabilizer.

Good luck with your upgrades, and don't go posting this on several different forums. Most of us browse through all the different forums, and will eventually see your post.