Tim,
I looked at that guy's listings--very interesting. He seems to be quite thorough, and to be offering good value.
The PR99 certainly could use some upgrading. I have one as well, and so far all I have done is replace the playback electronics card with one from Arian Jansen, and take my output directly from the PB card, bypassing the extra electronics of the balanced output amp. Of the upgrades that Mony offers, I am most interested in those that affect the transport: ball bearing replacements for the guides, and motor lubrication. My machine has a roller guide coming into the headblock and a fixed guide coming out of the headblock. I think replacing both of these would improve the sound quite a bit. But removing the friction caused by the fixed guide might necessitate readjusting the holdback tension to compensate.
Which reminds me: one of his kits includes replacements for all the trimmer pots on the cards. The stock trimmers are low quality, so replacing them makes some sense. But that would also necessitate a complete alignment of the machine, both transport and audio. Not a small task.
If you're just interested in upgrading capacitors for increased audio quality, you should search the forum here for previous discussions. They have some good ideas. But be aware that the transport mechanism has more impact on the sound than the capacitor complement. The PR99 transport is pretty good up to a point, but not approaching greatness. If you're going to all that trouble, you might be happier in the end if you started with a better transport. I'm about to replace the PR99 that's currently my home listening deck, and I have chosen an Ampex ATR100 transport and Bottlehead Eros electronics. Many people think the ATR machine is huge, but if you get it out of the stock casing and do without their I/O modules, like I'm doing, you find out it's practically the same size as a PR99 or many of the other decks that we consider "home-sized."