There are guys on here that run machine shops so probably better placed to answer this but at a guess I would think it would depend on intended use, budget & space. OK so I'm old school having done all my machining on manual machines & I have to admit to being a bit spoiled being in the RN because the workshops had just about everything.
Small mills seem to attract a lot of attention & sell well probably because a lot of people have a limited amount of space & are looking for a small hobby type machine, add to that the fact that they are normally single phase so plug straight in to most peoples domestic set up & can be shipped with relative ease & I suppose it's no wonder they sell.
Take a bigger industrial type machine that's going to cause more problems & expense to move, plus needs more space & either a 3 phase supply or someone who knows enough to run it off a single phase supply & the interest drops & you can pick up bargain machines if you look about.
If looking at a serious business machine then that's what I would be looking to do, not sure how easy conversion would be but would certainly investigate. If your looking at a hobby type machine that's more for your own use then a router might be a better choice. Speak to some of the guys on the forum who know what their talking about & they will tell you that routers aren't really suitable for machining any metals although you can use them for that sort of work.