There's a lot of answers to your questions. The best thing would be to get the NRA and CMP rule books, they'll explain exactly what you can and can't do.
Here's a link to the NRA rule book which will explain both service and match rifles; the CMP's book can be found on their website somewhere (I'm at work right now behind a filtering program so I can't look for it right this minute) which covers what makes a service rifle a service rifle (CMP competitions are service rifle only, match rifles don't exist to them...don't worry about that though, you'll shoot a lot more NRA matches than CMP matches during the year).
In general for a service rifle AR, it must conform to the same basic external appearance of the M16A2 or A4 (with attached carry handle) rifle. Things like collapsible buttstocks, ergo grips, picatinny rail fore ends, muzzle brakes and the like are illegal. Internal modifications to "improve the functioning and accuracy" of the rifle are okay, such as two stage triggers (no lighter than 4-1/2 lbs for a service rifle), free float tubes under the A2 handguards, and heavier barrels. For starting out, you'd be well served with a regular ol' Rock River National Match A2 upper. If your current lower has anything other than an A1 or A2 buttstock, ditch the stock and get an A1 or A2 stock and a two stage trigger like the RRA, put all that together and you'll have a legal, ready to go service rifle. I can squeak out decent scores with my RRA NM, just not consistently but that's my problem, not the rifle's.
Match rifles are much more free form as to what's allowed and what ain't. Calibers, barrel lengths, sights, adjustable buttstocks, it's just about no holds barred for match rifle shooters. You get a much more adjustable rifle (shooting a service rifle, you generally have to conform yourself to the rifle. Match rifle shooters can conform their rifles to themselves through adjustable buttstocks, cheek pieces, handstops, etc.), you can also get a significant downrange performance boost ballistically shooting a match rifle.
Shooting a service rifle, you're limited to your choice of slings. The only legal slings are leather or biothane 1907 pattern slings like the Turner NM sling either in standard or "no pulse" configuration, or the M1 web sling. Match rifle shooters, just like their rifles, are allowed a lot more leeway in their choice of slings, they can use cuff style slings and they can use a hook on their shooting coats to help hold the sling in place; you cannot use this hook (standard on Creedmore shooting coats) with a service rifle.
The rule book will cover a lot more than I have. There's a lot of good info on highpower specific sites too, like
NationalMatch.us and on AR15.com's CMP/DCM highpower forum.
