Check out the restoration section. There's a zillion threads about Donkey Kong/Nintendo cabs.....well OK, a couple.
Buying a 27 years old electronic device is always a gamble. You can't expect 100% reliability. Then again, if it works, it's not like it will break down every week or month. But you have to be able/willing to do some basic work on it when needed. There's heaps of info about DK cabs around on the internet and lots of people own one.
A replica maybe a nice project if you're looking for something like that, but for me (and most collectors I guess) that would be just that: a replica, a copy.
Anyway some info:
This is the original Nintendo Power supply set-up.

If there's a switcher inside, it doesn't have to be a problem, again it's just what you want. Do you like to have it as original as possible or can you live with some "hacks" ?
The monitor used in the vast majority of DK cabs is the Sanyo 20EZ. Looks like this:

Pretty easy to recognize with that frame construction.
Earlier (red!) cabs can also have the Sanyo 20C instead. It's basically the forerunner of the EZ.
My RS/MB has one and it's mounted in a wooden "box".
