What Xzin said tbh, go custom, I got lucky and found a nice desk top that suited my needs in a bargain corner, just had no legs/frame. Ikea do cheap legs that are fixed and I had some fittings lying around to make them adjustable, but Ikea do do adjustable legs too, and they're pretty sturdy.

Kitchen worktops are another route I looked at, but felt they weren't deep enough to get what I wanted on them.

You can buy the runners/brackets for under-desk keyboard trays, I had some from an old desk and used a 4ft (1200mm) length of wood about 400mm deep, have a 1ft long mouse mat on one side, Logitech G15 in the middle, N52 to the left and about 8" (200mm) left spare...

My monitor stands are the same Ikea legs actualy... bought cheap tiltable LCD mounts that are designed to be mounted straight to the wall, drilled holes for the bolts in the upside down legs, which are attached to small 250x200mm boards for support... works great, cheap, adjustable and expandable

I keep meaning to get pics up, but the 4th monitor is broken atm so I'm holding off on pics of the setup etc