Chris from the looks of it you really do need a Mill.! . . . Router style machines like what you have started designing don't really do cutting steel and aluminium so I'd forget that design.!
Router/(Weak)Mill can be done but needs a very different design. Steel and aluminium need a very strong dense machine which will absorb the vibrations.

You also have a slight conflict regards the materials and the spindle speeds they require for cutting.? Cutting steel requires low spindle RPM, this is also quite low for certain tools like dove tail cutters, which require sub 1000 RPM. Aluminium requires much higher speeds for best finish. The difference between the two can and will be several thousand RPM and while Aluminium obviously does cut at lower RPM's it much prefers higher speeds to get best finish and cutting feed rates.
If you plan on cutting high end Aluminium like 7075 then I suspect finish and appearance will become important so the relationship between feed rates, spindle speeds and machine strength plays a big part.

So the point to me saying all this is that you need a very strong machine and a mill will probably provide this best. . BUT . . due to your requirements of cutting 7075 type aluminium mixed with steel using cutters like dove tails then it will need to be one with a very good spindle providing wide speed range.? . . . These are few and far between and usually come at a cost.!!

Don't let me put you off building a router style machine and even with limited tools it's very possible to build a great machine. . . BUT I WILL . .try to put you off building one with the intention of cutting steel or high end aluminium unless very specific designed to do so. ~(I will also advise the cheap Chinese machines are not up-to the job either so save your money.! )

Sorry if this may not be what your wanting to hear but it's true and cutting steels or aluminium to a good standard requires a very different league of machine and unfortunately this does come at a high price compared to standard router type machine.!

Save your money and buy a Mill with decent spindle is my advice.!!. . . OR. . think long and hard before building a machine looking at a very different style machine than what you have been.

Good luck.