Just a caveat:
M-head came with three spindles:
B3 - just walk away. No, don't even look back.
JUST. WALK. AWAY.
Collets are almost nowhere to be found and the chances of finding other spindle tooling like end mill holders is zilch minus ten.
Unless you have the capability to regrind this to something else, you are probably screwed. Even if it comes with a complete set of collets, you can't get anything else. Hell, if you get it cheap enough, part it out and sell the collets one at a time for outrageous prices like MikeKandu and use the money to buy another machine :-)
B&S7 - better than B3 (how could it possibly be worse?). Some tooling is available, but you will spend a lot of time and money combing ebay for tooling and I don't need anyone else bidding on the parts I'm chasing.
Seriously, I have spent more money on B&S7 tooling than the difference I would have paid by getting an MT2 spindle head.
I DID however, pick up an MT2 spindle and one of these days I will swap it out and sell the B&S7 spindle and tooling to some sucker that bought a B3 spindle.
MT2 - you can get anything and everything in MT2. Everything from top-notch USA-made tooling to cheap imports and anything in between. Collets, end mill holders, milling arbors, taper shank drills, boring head arbors, whatever.
Steve