Not trying to discourage in any way, but I have a few points when buying a Mill.
Everyone is talking power so I'll leave that alone.
No matter what you buy, you need to ensure you have excess in your budget for Collets, Cutters, and a Vise. Depending on what your plans are you can get a set of cutters from Harbor Freight that will do sufficiently on aluminum. Collets, here is a good portion of money you need to spend. R8 collets are pretty much the standard for most bench tops. You'll need any where from 5 - 8 depending on your cutter shaft. commons are 3/8, 1/2, 5/8 and a few more. If the mill doesn't come with digital read outs, I'd highly recommend buying a set and installing them, as the slop in the cranks is always there and only gets worse.
A Vise, for most table top mills a small vise is all that is needed. But if you end up getting a bridgeport or any type large knee mill, a decent vise will run $200-$400 or more. A full set of mill clamps (T-Nuts, studs/nuts and clamps) can also add to the budget.
Someone asked about a knee mill, Katy ISD has a JET knee mill in the STEM Center for the teams to use. I know 624 (CRyptonite) and a couple other teams used it extensively this year. I can provide further info.
Have fun and happy cutting
