[diarmaid]

Hi. I dont have personal experience about this but these might be able to help:

http://www.machinetoolhelp.com/Purch...nspection.html
http://asuwlink.uwyo.edu/~metal/makers.html
http://www.machinetoolhelp.com/Purch...chinetool.html
http://www.machinetoolhelp.com/Purch...oreyoubuy.html


A very thorough discussion of vertical mills and what to look for was in Home Shop Machinist, July/August and September/October 1993, by Thomas Howard. Here is a very brief summary:

Spindle-to-table distance, or "daylight." Remember that by the time you clamp your work in a vise on a rotary table then slap a chuck with a drill into the spindle, you might run out of room. Try to anticipate your needs.
Knee mills: A mill where you can raise/lower the table with a crank, as opposed to only raising/lowering the head and/or quill. Knee mills are generally more useful and accurate.
Spindle brake: Locks the spindle during tool-changing. Very nice to have.
Power downfeed on spindle. A very nice option, relieves tedium when boring, and often yields better surface finish.
Spindle taper: determines what type of tooling you can "plug in" to the spindle. R-8 is most popular, and there's lots of inexpensive tooling. Morse and Brown&Sharpe exist and are less popular.
Bearings: Apparently it's not as important whether they are ball roller bearings, as the grade. And the better the grade, the longer the machine lasts, more than anything else.
Table sizes and travel: Like lathes, you can always use one an inch larger! Don't buy one larger than you have room for. From 6x20 (inch) to 8x30 seems right for most home shops. What will you be doing with it?
Power feed on X/Y axes. Very nice to have, just barely falls into the "luxury" status.
Table and knee locks: Used to maintain rigid setups when one of the axes won't be changed during an operation. Check for easy access and that they lock solidly.
Graduated dials with adjustable zero: an absolute necessity.
Adjustable gibs: a necessity.
Range and number of speeds. An average is 100-2500 rpm, and 12 to 16 speeds. Howard goes into great depth in his article to explain why you need both a wide range of rpm and many speed, and how those speeds should be spread out in a geometric progression.
Motors: be sure you get a motor you can run. Check frequency (60 or 50 Hz), voltage, and 1-phase vs 3-phase. For home-shop use, anywhere from 3/4 to 1-1/2 seems right.
One-shot oilers are nice but in many cases they don't work right and are hard to check.
Look. If at all possible, examine the machine you want, before you buy.
Buying used: you should be experienced enough to know what to test for! Or "borrow" somebody who is.
Will it fit? Be sure it can be moved to where you want it. Don't buy then find the stairwell is smaller than you thought. Get professional movers to do the moving; "real" mills are literally "killers."
Tooling: can sometimes double the cost.

Im sure someone experienced will give you more advice, hopefully before you go to look at it!