I have an X-700 too and I adore it. Great glass is plentiful and cheap, the body does everything you could ever hope for and is reliable as you could want. As Eric said, if you are looking for glass, check out Antony Hand's website: the Rokkor files.
IMHO if you are looking for lenses, be careful with eBay. Just remember, there is an absurd amount of excellent MC and MD lenses out there, you can really pick and choose from the most reputable dealers. FWIW, if you do comparison shopping with KEH online, you'll find that their prices are on par with eBay and you get a guarantee.
If you are looking for zooms instead of primes, you can't go wrong with the 75-210 f4 and the older 35-70 f3.5 (not the newer variable aperture).
There are really only two real problems that come up with the X-series and both are easy to correct. The first is degrading light-seals which is an easy fix and the second is failing capacitors. This is an easy fix too.
Hope that this helps,