Chaos knows these guns pretty well.
They're big, loud, heavy. They're made of heavy machine-grade 6061 aluminum and 316 stainless in most places, so they last a long time and can be used as hammers, jacks, or pilings for small buildings.
While parts aren't as available as they used to be, they also aren't needed super often. O-rings are common and can be ordered through plumbing suppliers (cheap rubber) or decent hardware supply (teflon or other long-lived items). The cup seal can be substituted with an autococker item in a pinch.
The hard to find items are the seal on the front of the hammer (doesn't go bad often, though) and sear parts (if the sear is worn out or trigger assembly is bad, you've got a problem for a VM-68 guru--they're fiddley the first time and you WON'T get it right without help...if you can find the parts...). Detents ("magic fingers") are $20 from Palmer's or can be cut from gasket material.
These have hit their low value point, so they won't be worth any less if you try one and don't like it. Look for one in good condition, the $35 specials are often too rough to consider, but you don't need to wait for a $200 NIB special either. They're common and a $60 tiger-stripe in playable shape will come your way if you look.
And there's more than a few of us who can give you advice...these may be old, but they aren't forgotten. Heck, I was using one just a couple weeks ago.