I kinda had some of the same concerns as you initially, goon - as far as how available are replacement parts, as well the post-151 grip frame change. After a bit of research to see if the parts and grips are out there, I was pretty comforted. If I do buy one, I will limit myself to the 151 or later serial number pretty much just because of aftermarket grip availability. As for other parts, I feel pretty safe.
As to your earlier comment about getting near the price of a new GP100 or SP101 - that's definitely a consideration I had. But all the new GP100's and SP101's I find still tend to be closer to $500. Aside from that, I don't care for the blockiness of the GP100 (as I intend to use the Six for CCW), but also prefer a full size 6-shooter to the SP101. The major appeal of the Security/Service Six to me is that it offers similar feel of a K-frame Smith, while being stronger than the K frame; is less bulky than an L-frame, and simply more affordable to me than a Smith.
I've decided I will walk away from the one I was considering buying at this time. I've had my eyes open for a 4" stainless Service model for awhile now. Had one in very good condition offered to me online for $390'ish (before shipping/transfer fees, which will combine for me to the tune of $50). After doing some more research on what similar condition guns have sold for recently and hearing some folks here, I think I will just continue to keep my eyes open, and jump on a better deal when it comes around.
Thanks again folks!