Hey, another Cyklon packrat
But it's nothing to be ashamed of! They're great regs, and if you like how they work it's hard to use anything else. I have 2 old 300s, one 5000, and a few 2nd stages.
The main issue with the Cyklon 300 is that the 1st stage isn't balanced, so it has a huge IP swing on a 3000 psi tank. I find it breathes best on low pressure tanks - it's nice on steel 72s but kind of stiff on aluminum 80s. I wouldn't even think of using it on a high pressure tank! Also, the hose fittings are weird but you can get around that with adaptors. Otherwise it's a nice little compact reg and worth rebuilding if you can get parts.
The metal and plastic 2nd stages are slightly different - the mouthpiece is a lot wider on the metal ones. This actually effects the way they breathe, and I find the old metal design is a lot nicer. Other than that, they are identical and use the same parts. If you don't want to rebuild the old 2nd stages you just bought, you can always just swap the bodies out with your current ones.
Cyklons use non-standard LP seats, so you can't completely rebuild the 2nd stage using generic parts. Other than that, the orings in the 2nd stage aren't very critical anyways and shouldn't have to be replaced very often. Off the top of my head, the 2nd stage swivel is a standard 010, and I think the orifice takes an 008 (but don't take my word for it). Also, be aware that Cyklon diaphragms are quite expensive, and if your yard-sale regs are old enough they will probably need new ones.