Modding is quite popular around here. Personally I've not had good experiences. One was a battery powered amp. The battery required shipping back to the modder for periodic replacement and sure enough it was damaged during shipping. Come to find out, the build quality of the mods were "suspect", so the heavy battery was bound to come loose and tear up the insides. The other was a CD player that I bought used. Within a few months it would produce nothing but terrible distortions. The modder would only suggest fixes that I would have to do, with the parts costing $180.
Underwood Wally is definitely one of the better known/respected modders. Would it come with a warrantee? Another issue is just how much (percentage of the stock price) should one spend to mod. That's an individual judgement call, but there's no point in modding something that is poorly built to start with.
Buying used is another question. CD/SACD/universal players are the audio component most likely to brake. I had a player from the 80's fail when the plastic parts in the drive section wore enough to no longer allow for the laser to track.