Basic parts kit (springs, a buffer and such) is about $125 last I checked. That's where I'd start. Buffers seem to be the thing that wears, though after thousands of rounds through mine, I've yet to wear one out.
Personally, though the bolts never break, the firing pins sometimes do and it's much quicker to swap a broken firing pin via just changing the whole bolt than it would be to break down the bolt and install a new firing pin.....they run about $125 as well. But I'd also buy spare firing pins just to have....there are upgraded Titanium ones on the net. No idea as to quality.
I have a 3400 and 3700 SN range XCR-L....both had tight chambers (among other issues) and wouldn't run steel cased ammo without getting them stuck in the chamber on a FTE. If you have that issue, rest assured....we can get it fixed provided the bbl is at least 10.5" (and I'm pretty confident even if it was around 10" we could get it going...... I'm guessing that's irrelevant though as I'd guess yours is a 16" version.)
We've seen one person break a carrier, but that's obviously not normal.
That generation of XCR had a two piece welded op-rod. Sometimes, rarely, they break at that weld....one piece ones are available to upgrade if you feel the need or just want a spare.
If you plan to run bi-metal bullets, might be wise to invest in a spare bbl as bi-metal wear the bbl faster. Problem is: bbls are expensive from RA for what you're getting in terms of quality. IIRC, around $400 with the gas block and dial.
You can upgrade to ambi mag release if you want, better bolt hold open design, ambi safety and the "enhanced trigger". IMO, for the money, the enhanced trigger is the most bang for the buck.
You can file/grind the tab on the upper upon which the front pin rotates if you want the rifle to open wider in the event of a bad malfunction that would necessitate easier access to the trigger group area for a blown primer, etc.
Obviously, you can upgrade the stock...which it looks like you're already in the process of doing. There may be a market for the things you take off....I've bought the tube folders in the past for potential AK builds as the quality is better than most AK offerings.
You can even upgrade the gas system, bolt design and buffer to the latest configuration, but frankly, I like the older bolt/buffer configuration. Machine work would have to be done to your lower to upgrade to the new gas system and bolt/buffer design though. Personally, I think the old one is better if you don't plan to run suppressed....but that's merely my opinion.
HTH,
Sean