As for the round column mill, they can work fine, but with the limited Z-axis travel, you will wind up wishing for a dovetail column machine. Consider that the Z-axis travel has to be sufficient for the part height, and the tooling height differences, and for clearance for tool changes. Say you go from a drill in a chuck (long) to an end mill in a very short holder. The Z-axis travel has to soak up the difference in tool height before it can start milling with that end mill. Also, you need height to r&r tools from the spindle. With an R8 toolholder you are going to need several inches clearance to do that. On a dovetail machine it isn't a problem as you just raise the whole head, but a round column you may not have enough room. This can be alleviated to a point with tooling choice. Something like the Tormach TTS system reduces the room needed for tool removal, and you could use a collet system to keep all of the tools close to the same length. It can be made to work fine, just wanted to add some things to consider.
For parts 50x50x15mm, have you considered a BF20/G0704 size machine? Cheaper to CNC, lots of support for them on here, dovetail column, etc.