I have a feeling your budget is way too low, even for decent work in aluminium, the likes of the 3040 on eBay will do very light cuts on ally, not steel, but will push your budget and fail after a short time. I had thought of getting a 6040 from eBay but after research I decided to build one better, the budget went out of the window, from £400 to over £1500 now but I am far more confident of results and life expectancy.

The Proxxon is a mini drill really, don't class it as a mill, for watchmaking maybe yes but anything bigger no. If looking at other smaller mills to convert, try not to look at those with swivel heads - these are very weak (i had one).

As said, what is your target project, if we know the subject we could guide a bit more :)