For spare parts it would have to be a full replacement kit of springs, followed by firing pins, then an assortment replacement of screws and roll pins if one so desires.
Ebay can give one many spare parts for cheap cost(can't sell/purchase the serialized receiver without a FFL involved) when it comes to HP's and this would be a great way to build a HP tool box from, for pistols and carbines.
Just remember to stock up with some extra magazines(factory and aftermarket) and lots of ammo that goes bang. Always have a box or two saved back for emergencies and always keep a couple extra magazines loaded. Just unload, clean/oil(dry Teflon spray) miminum every 6 months, unless in dusty areas to be sure you always have working mags for those emergencies.
Don't really know if ammo can go stale over the years in a good dry environment and moisture proof/sealed ammo cans like I have, but if it can, just rotate the stock and you're GTG everytime.
I say this because I have reloaded shotgun shells and rifle shells that my dad reloaded 30+ years back and they all still work. And they were in an environment worse than what I have all my most recent store bought ammo in.
Del