$50 for buy it now actually seems like a pretty decent price if you actually want it now. Assuming the other one doesn't go up in bidding, $20 would be a steal.
In my limited experience, the older the better. I've only handled two 1989 and one 1995 specimens. Both of the pre-90s boards felt better, even after a partial bolt mod on the '95. Not sure how much of a difference there would be between 1988 and 1990 though.
As others have said, there are quite a few cable options. I like the orihalcon SDL to USB cable, but you can also get new PS/2 cables from Unicomp, I think. Or you can do a USB mod and use any type-B USB cable.
Probably the biggest issue with ebay is that you won't be able to see how many broken rivets the keyboard has unless the seller has the right tool to disassemble it and photograph that.