The typical t-slot-extrusion mills are often called "3040 routers", because the table size is usually 30 x 40 cm.
I don't have one but people who do say that the cheapest ones aren't really precise enough for anything more than artistic use, since they only use one acme screw for each axis and the (cheap) slides wobble. They might work well enough for copper-clad routing, but they're not accurate enough to make machined metal parts.
If you have access to a drill press then you can buy better parts than that, but it's not hard to spend over $1K just for the metal parts for a little machine.
I have a benchtop mill and benchtop lathe that are both manual. For various reasons I don't think they're worth converting to CNC.
I've seen a few of the typical "router bits" that are sold for CNC routers--I've ordered some out of curiosity and I've gotten some for free when ordering other stuff--and they're all made very poorly. They are real carbide, but the grinding is usually done very badly. The cutting edges don't meet properly and they are totally unusable for milling metal. Sometimes the cutting edges are left totally unground in spots.... A lot of the problems people have with crappy CNC router results could be helped by better bits, but the china bits cost $1 and good bits cost ~$10.
One thing I've always wondered is why you can't use a CNC mill also for 3D printing. Nobody online seems to be doing it. A 3D printer isn't stiff enough to work as a mill, but a mill should be able to do 3D printing with just a change of spindle attachment.
The main reason I've heard people claim is that "a CNC can't move fast enough for a 3D printer", but then,,, I've also heard of people printing big & complicated stuff that takes 12-18-24+ hours to print. How fast can the thing really be moving if it's taking that long? And extrusion printing with thermoplastic has an inherent speed limit in how fast the plastic can heat up and cool anyway. I have a hard time believing that such a dual-use machine would be impossible to build inexpensively.