There are some I-frames going second hand at around £100 at the moment I think, well worth getting if you can beg, borrow or steal the money. Delrin bolt is always worth it, but remember to change the hammer at the same time. Really decent barrel is also important, I prefer the Freak because of the flexibility.
But the best upgrade you can do, unfortunately the most expensive as well, is getting the Vision eye fitted (though I'm not sure if you can on the very old ones).