Y travel of most of the desktop machines is a stumbling block but it's the price you have to pay for having a desk top.

Space needed is usually the footprint of the machine plus 1/2 the X travel added onto the bed size. This is theoretical and in use you need a bit more for bits hanging off.
Many people stand a mill in a corner at 45 degrees to take advantage of the back part never having any movement and gaining the bed travel as it sticks out, also possible to still have benches / cupboards to either side and clear parts off when using the mill.

Support is more important on a CNC mill, ask question on how they deal with support and spares once bought. Is the machine you choose actually supported in the UK or are you tied to a US web site ?

Deep subject requiring a lot of background.

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