I recently picked up a mill of this type (Mid-range, good for anything from wood/plastic on through copper or aluminum) after looking around quite a while. I checked Alibaba and couple other places for deals on Chinese mills but ended up sticking with eBay when it came to buy. It has a _much_ better structure in place for pricing, communication, and seller accountability if things go pear-shaped. It's also good for getting an idea of how trustworthy the seller is and how long they've been active, which is a big deal for me.
You didn't state your price range but I was looking in the neighborhood of $400-$850 (USD) and had a number of things that the mill had to have: ball screws, solid aluminum frame, and a fourth axis. All the generic Chinese lathes tend to use the same type of nomenclature when describing these products: size in cm for X and Y axis, like 3040 for a roughly 30cm x 40m usable area. They also seem to come with more or less identical parts (At least this was the case for the cheaper ones I was looking for), with the exception of the size, whether it had trapezoidal/ball screws, fourth axis, and type of spindle motor (If provided at all). Another nice thing is that there seem to be a large number of people reselling these mills in California so they usually provide free shipping and it arrives within a week or so. Just a warning to be prepared and have a way to move it (Like using a furniture dolly) when it arrives since the main box mine came in was a large, wooden crate that weighed close to 100 pounds!
In the end I went for a 3040 size instead of the cheaper 3020 size, it had the desired ball screws and a very solid fourth axis option (Came with a lathe-style 3-jaw chuck and very hefty motor/gear/chuck mount); it was $707 with free shipping. As far as a spindle motor goes it came with a 560W 120V motor (Supposed to run at 15,000 RPM) with a 3-foot long flexi-shaft cable that served to couple the motor to a small mounting plate/connector on the mill with a mini drill chuck for mounting bits. Not exactly ideal since there wasn't a neat and tidy way to arrange the motor, you just had to set it off to the side and avoid blocking the air intakes/exhaust. It had a fairly average runout, not terrible but not perfect, and I've been working on getting a proper spindle motor that will run at the high RPMs required for PCB milling and drilling. You want to be careful to get a high-quality spindle motor since otherwise you stand a good chance of snapping carbide drill bits and having your trace accuracy destroyed by a milling bit that is not stable/centered.
I upgraded the stepper motor drivers once I used mine for a bit since I found that they tended to "jerk" when stopping or starting, as if the first or last step required a large jump to get into position. There are a large number of cheap drivers available for 1 or 2-day shipping on Amazon and I picked up some inexpensive replacements for about $15 each for a 4A driver with up to 1/32 microstepping:
These work really well and produce extremely smooth, quiet movement at 1/16 stepping. This provides a very high fine resolution of over 40,000 steps/inch, which is more than enough for my needs. Having ball screw lead-screws helps to avoid a significant amount of backlash and I'm very happy with the testing I've performed, including mounting a 250mW 410nm laser to cut paper, mark wood, and expose photoresist. I don't have any testing yet for PCB milling due to the lack of a high-accuracy spindle, but I'm working on that. Here's a listing for the same mill ($709, free S&H) and seller I bought from:
Best regards,
Larry