I owned the ML 23 at one time. I thought it sounded very good although it was not an easy amp to move about.
If I were buying one that has been worked on, I would want to know who worked on it and if they are truly competent when servicing ML gear. I would also want to know what they specifically did to the amp.
One of the best places to send ML gear for repair is Pyramid Audio in Texas. From their website, here's what they have to say about the 23/23.5:
No23 and No23.5 have amazing build quality. Most problems have nothing to do with a design weakness but are simply due to their age. For almost 30 years now they have cooked themselves to the point of failure. Service is difficult because the internal construction has no connectors. That’s right, everything is soldered in place. Not only that but the common problems are not so much capacitors but a fair number of resistors that have aged and have become noisy and there are a number of capacitors that need to be replaced. The good news is the very largest electrolytic capacitors almost never fail and that does keep the price down to some extent. However the repair takes in the neighborhood of 30 hours. The result is an amplifier with the original specifications and classic Mark Levinson sound. So the bill for overhauling the No23 or No23.5 typically runs about $3000, maybe a fifth of the replacement cost.
I would stay away from any 331/2/3 amps that haven't had their power supply caps replaced. The originals were very problematic and the repair is not cheap.
As with any vintage amp, I would look for "purpling" of the anodized metal of the heat sinks/case especially on one side/section of the amp. That means the amp was running hot. Let the amp play for 15-30 minutes and put your hand on the heat sinks...are they warm and symmetrically so? If not, some of the devices might be working harder than the others, which is not a good sign. You might call Pyramid and ask them what the temperature should be for a properly working 23.
If you do your due diligence, a properly working 23 for $1500 would be hard to beat.