Router bit recommendation, spindle speed and feed rate?

[Jonathan]

The straight flute hand-router bits I have are certainly not cheaper. I like Trend cutters

I suspect they are if you compare like with like - i.e. compare trend prices to companies in England/USA selling carbide spiral cutters intended for CNC.

Anyway, you recommend spiral cutters for CNC. Should I use a different type of bit for routing hardwood / MDF than for aluminium and other soft metals? I'd intuitively expect metal cutting bits to take a more shallow angle of cut.

It depends on that many factors we can't really recommend without more idea of what you're cutting. Balsa and oak are both hardwoods yet clearly will be treated differently.
In general on a CNC router single flute carbide cutters are the best for aluminium/brass since only one flute leaves more room for the swarf to clear. This helps prevent re-cutting of the chips, ultimately resulting in a higher feedrate and better finish than other cutters:

http://www.ebay.co.uk/itm/5x-6mm-Car...item20b95dbb6d

If you're looking for one cutter to do everything those are the nearest I've found since they also work well in plywood, nylon, polycarbonate, copper, pine plus some other hardwoods I've cut but don't know the name of.

Is there a worthwhile difference using HSS for softwood or should I just stick to carbide?

Definitely use carbide as it will last much longer. That's especially true for composite woods, like MDF or plywood where the adhesive makes the material very abrasive.