The only parts you "might" run into problem with are rear suspension parts and if you have bike the manufacture doesn't support anymore for some reason a lot use standard bearings. Breaking a linkage or swingarm though could be the end if you can't find a used replacement. Older bikes are more likely to have straight steerers and highend replacement forks are getting hard to find with straight steerers. If you go with a 26er new highend 26er wheelsets are getting harder to find, forks are really hard to find. Nice QR wheelset are also a bit of pita to find but a lot can still be converted to QR so not a huge deal. Went through that probably two years ago for my 2011 29er and even today that ancient full suspension bike is still worth upgrading/repairing.