I haven't posted in MONTHS but know a little about these so thought I would share.
On price, I wouldn't expect it be anywhere near current prices for used LP2's unless there is a crowd that finds these collectable (or are simple uninformed).
On parts, there are certainly some differences as models mature and I don't know for sure but I'd bet 'most' modern small parts would still fit. Obvious exceptions would be the block and loading gate and the co2 and or compressed air adapter. Yes they can (could) easily be swapped from one to the other with the right adapter but they are/were different cylinders. Bulk filling a co2 cylinder isn't an expensive undertaking but you'd need the stuff to do it.
That said, using Steyr model names, new - the LP1 was CO2, the LP1P was compressed air.
I'd pass. If you want to explore basement shooting, start with a 1377 or a P17 until your convinced one way or the other. I shoot my LP2 for a couple hours a day if I'm bored but I'm only okay on target. Results at target are not $1000 better than I get with my P17, but I still choose the LP2 first.