There's a current standard service kit for the G250V/S600 that includes a seat and a bunch of o-rings. If you can find a slightly older kit, (ebay or a low-volume dealer) it will include fewer o-rings (you don't need all the extra ones anyways) and a poppet and balance chamber. More likely, you'll need to buy the poppet and balance chamber separately. I would also recommend getting a new spring (probably $5 max) and a new lever. The lever you have might work fine, but there are a few instances in which the old lever and the new poppet don't work well together. In my humble experience, replacing the spring has helped getting the regs to hold a tune more precisely. The springs probably wear out a little and lose some elasticity. It's nothing you can see, I just found that a new spring helps me tune it better.
I've never heard of a kit for a "pre-1995 G250" but I guess what you mean is that yours has the old metal-stemmed poppet. If I had a reg like that I would not even buy a kit, I'd get the seat/poppet/spring/balance chamber/(maybe) lever from the dealer, and get the few o-rings you need from a source like o-rings usa. The seat is really the only part of the kit that you can't buy otherwise, and it's about $4 as opposed to (probably) $20 for the kit.