The M head is really old. It does not accept R-8 collets, which are standard for almost all manual milling machines. It's maximum collet capacity is 0.5" without ordering special tooling. It also has a very weak motor (only 0.5 hp).
Have you seen the mill personally? It's likely been used very hard, due to it's age.
Some things to look for:
Rust. If there's any kind of heavy rust or pitting on the table or ways, avoid it like the plague.
Inspect the table. Is it in good condition? Have people drilled a million holes into it and cut slots through it? Is it flat? (You can check this with a dial indicator, assuming the head is square to the table)
Inspect the ways. They should look like
this . This is clearly a very old machine, so they will likely not look nearly as nice as the machine in the picture. You can still check them to see if they're highly worn/pitted/etc.
Run the table to the limits of it's travel in all directions. It should move smoothly and freely throughout it's travel. If it gets drastically harder to move in certain spots, it has major problems.
Check the backlash. All manual mills will have backlash, but you want as little as possible. Our Bridgeport clone has about 0.005". I'd be afraid of anything with a lot more than that. The backlash should also always be a consistent amount. If it varies from movement to movement, the leadscrew and/or half nut are probably shot, which would be a major pain for you to repair.
If it's under power, turn it on, listen to it run. If it's making funny noises, the spindle bearings and/or motor are probably bad.
Ultimately I think it comes down to you either know what you're looking for and can evaluate a machine for it's worth, or you can't. It's very difficult to tell what condition a machine tool is in just by it's outward appearance. Often times previous owners will slap a coat of fresh paint on to cover rust or make a badly worn machine look new. Or they'll sand down the ways to remove rust, pitting, etc. Other machines will have been well cared for and just look old, but perform much better than the cleaner looking machine.
If the cost is substantial, and you don't really feel confident you can spot a lemon from a gem, I'd see if you could find a machinist to go with you and look at it, or avoid it entirely. If it's really cheap and you have the means to transport it, there's not much risk involved, even if it turns out to be a beater.
-Cory
P.S. I came across these articles awhile ago, and thought they were helpful. One is for mills one is for lathes, but the lathe article is relevant to buying any kind of used machinery
lathe mill
[edit] I have read good things about
this DRO system on some machining forums. If you're handy with electronics, I've also read good things about
this DIY solution