As far as systems go here a re a few things to consider with all of them;
PCE Briefcase setup (PCE+CDROM+IFU); You can generally find these units cheaper, but you will almost always have to replace the gear on the CD-Rom itself, a cheap fix that can be easily done. These rarely have issues with capacitors going out as they are of higher quality than those used in the standard PCE Duo, but sometimes they fail, it's just very rare.
PCE or Turbo Duo (black combo system); These are notorious for caps leaking and you will likely have to replace caps no matter what. The caps used in these are extremely cheap, if you find one of these systems in the wild that has not been serviced then chances are the caps are leaking all over the board.
Super CD-Rom2 system (Grey CD attachment for standalone consoles); These also suffer issues with cheap capacitors leaking as they appear to use similar parts to that of the Duo.
PCE Duo R/RX (fat white combo system); These rarely need to be recapped. I have had 3 altogether and not a single one had ever been recapped and have gone to new owners and afaik are still functioning. I would not mess with caps unless your sound is really off. You could always open them to see for yourself if they are leaking or not. This was very visible with one of my regular Duos and especially the so with the Super CD-Rom2 system I had.
I do shows and have had a couple of my systems serviced to show off our game and the guy I used does a good job at recaps and RGB mods. I had my SCD2 system RGB modded and I use a SCART cable with it direct to whatever upscaler, but can still do composite out of the same port if need be to connect to some old TV's. Here's his FB page:
https://www.facebook.com/pcenuggets
He had s site I think but I can't seem to get to it. Just throwin' it out there in case you don't want to do the tedious work of soldering and wiring yourself. Hope that helps!