The other thing is how much space do you have? Are you looking for something bench-top or a floor mount? Spindles and routers are noisy things so something running for hours on end needs soundproofing or operating away from the home!

Well 100mm (4") diameter rules out the tiny micromills as they have a travel of around 60mm on the Y-axis. As Robin has already intimated the key is rigidity. Aluminium is harder than Perspex, but perspex has its own issues (and any reason why perspex and not an alternative engineering plastic that machines better; I guess transparancy is part of it, but there are other acrylics that machine better).

Cutting from the A4 sheet suggests a working area of 300 x 250mm minimum, a footprint of 500 x 400mm would be possible. 10mm depth of cut isnt an issue if done in multiple passes. Perpex needs a high spindle speed, and high feed rate to ensure its cutting and not rubbing else things melt. Some form of mist coolant is also needed. All these add ££ to the budget -

Router's dont have to be flimsy things (Robin is an old-iron man lol) but its true that getting the rigidity to machine hard stuff to fine tolerances is hard to do - fixed gantry moving table is more rigid but has a smaller work area than a moving gantry design for the same footprint and slide rail/leadscrew sizes.

Machining a PCB is relatively easy on a gantry type router, but very fine lines are best etched so dont be expecting to do PCBs for that all-singing PGBA package but basic through hole or SMT stuff works fine. HAve a browse round the CNC mill videos on YouTube to get an idea of the sort of thing.

Some good devices have been constructed from MDF (£) although the preference would be aluminium extrusion (££+)

A common route is to construct a learning one from MDF and use that to a) gain experience and b) make parts for Mk2

For plans look round the web; JGRO, Solsylva, Buildyourcnc.com, mechmate and rockcliff are typical designs. I cant recommend as I've not used any, but several members here have built to these designs - or more usually used them as a starting point...