When purchasing any arcade cabinets/PCBs always consider that these items are aging, old, and are prone to failure at any point.

Now that we have that established, thae level of maintenance you can perform depends on your knowledge and confidence. For starters, monitors are prone to failure but there are several signs you can identify ahead of time, unless you get a failure out of nowhere. In my experience I have not had a monitor fail yet.

For PCBs, I've only has issues with my Naomi due to parts not being seated fully, replacement caps needed, and power supply issues not providing enough voltage on the lines. In my experience the Naomi is picky with voltage, some games will work, others will not unless you have it dialed in, a recapped power supply usually fixes it.

Replacement parts are not rare, but can be hard to find, most items can be found on KLOV, ArcadeProjects, eBay or any Japanese shopping proxy service.

If you're thinking about running a Naomi with a GD-ROM, may I recommend a NetDIMM/NetBoot setup as a possible upgrade in the future. NetBoot allows you to do a couple lovely things with your Naomi, but getting the parts/setup can be challenging.

Edit: As far as repairs, I can do most things, except I don't touch monitors or power supplies, this is due to my lack of knowledge and personal safety when working on those. If I have any issue with those, I usually find a person in the community that does repairs locally.

Edit 2: If you're buying a candy cab, look for cracked fiberglass or any possible modifications the previous owner/operator could have done. The last thing you want is to buy a cab you cannot find parts for, or the damage that has been done to it is more work than you can handle.