Build it up. But take your time and try to get the best deals for everything.
If you dont mind the wait, there are good deals to be had. New other is a good source of bits on ebay. always people buy things as an impulse then sell it off when they find they dont actually need it. Other than that sales on Merlin cycles, alpkit outlet or even tredz can save you a lot of money.
I also bought one of their carbon mega frames and built up a hell of a good bike. Fox factory, hope pro5, tech4V4/vented,GX full groupo for about £2k using the above sales, new other etc. Everything I got is new, so maybe no frame warranty, but still an amazing build for the price of a mid range hardtail
Just keep your eyes open and spend a bit of time paroozing ebay and the bigger bike bit sellers and the deals will pop up. ‘Newly listed’ ‘new other’ on ebay can provide some immense bargains.
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As to frame spares. Because most of them were bought at serious reduction from crc, people(being people) list at top prices, but those will come down and in a couple of years you will be able to get spare stays etc pretty cheaply.