Hey Dredd you run at Floyd Bennet too right? I was in your situation once and bought my MRX4 package at my LHS a while back. It costed me $870 or so for everything minus servos as I got some basic stuff. The motor though was old (novarossi top PT21 SP1), but when tuned properly allowed me to run with anyone else out there. One thing was certain was that the tires on the 8th, the stocks and the Spider tires, lasted me a heck of a long time. With less power than the others I didn't have as much wheel spin and the way you drive also contributes to tire longevity.
After running my 8th and sedan, the 8th stood upto crashes a heck lot better than my tourer. So far I only busted a knuckle after being hit by a beginner, and toasted 2 rear belts at BPT track which is notorious for debris lodging in the rear hubs of 8th cars. The only amount of $$ I spent on my 8th was for one set of tires, a JR servo, and a few parts that I thought was more beneficial to buy. My sedan, which isn't a mugen, nor was hopped upto max factory specks costed me more $$ in the long run. Sure a Mugen sedan is tough too, but it's not indestructable. I seen others get into wrecks, but just not often.
As said both can cost about the same, but on a small track, a sedan will be cheaper as you don't have to dish out a lot for a powerplant. At the regionals I saw guys running stock .12TZ and ran with everyone else some making the A main even. I ran the OS .12TR and made 8th in the B main for touring that day. Not bad for the weakest motor on the field.
I also try NOT to break anything and if the car seems to be sliding towards a board or lost traction in any way, the brakes are applied...enough to keep me from being hit. You can always catch up later on in the race and regain your position.
One thing though, that RTR XR8 will cost you a lot to upgrade to, so why not get the pro kit? It's under $400 and very reasonable. In BPT CT Aces stocks plenty of parts for that car and even the LW alloy arms which I saw few guys running. Dean (BPT DEAN associated with Bpts kit club), runs that car also with some of those parts and I must say his XR8 is down right FAST with the Vega Scud .21. The narly wrecks we had too practicing there did nothing to his car. The XR8 is definately a car to start in 8th, but if parts are limited, do it right and go with one of the Big 3 (Mugen, Kyosho, Serpent). I run a stock MRX4 and like it a lot. I may not even change to the R specs even, but it'd be something to try as everyone who went with them had all positive feed back. Both classes can be expensive, or inexpensive, but overall you will need a tire truer and setup system for both. I'm debating on what car to run mainly this year myself as both are ready. The sedan still needs to get it's setup gauges and the 8th needs a race motor as I'm buying a practice motor from a friend. Whatever you do, don't do like I did and run both classes on one day. Never had to hustle so hard, but I enjoyed every bit of it.