Hi, just starting myself. However what I can tell you:

1A. Wood routers are going to put out a *lot* of sawdust. It will get into everything, including the linear rails, ball screws, etc. unless you handle it. If it gets wet, you will have mold and mildew problems.

If wood gets oily, it gets stained and difficult or impossible to apply a finish.

Sawdust will stick to oil and grease.

1B. A mill for steel is best with mist or flood cooling, at least cutting fluid, and will be oily and/or greasy. Flood cooling requires capturing and recycling the coolant with a pump and filter. Now imagine that pump and plumbing full of wet sawdust.

2A. Wood routing uses e.g. a 24,000RPM router, at relatively low torque.

2B. Steel milling and drilling requires closer to 50..2000 RPM (higher range with carbide tooling, slower if you ever need to use HSS) and MUCH higher torque.

Higher speeds for aluminum.

I will suggest looking at some mill cutters and doing the calculations for the desired operating speeds for the specified SFM (Surface Feet per Minute) or metric equivalent. That will tell you what speed range you want.

Also look at an existing milling machine that does what you want, or its manual, to get a sense of how heavily it is built and the speed range and spindle motor power.

3A. Wood workpieces are commonly up to 48" x 96" sheets or perhaps logs.

3B. Steel workpieces are usually up to several inches on a side, except for sheet metal and shafts which may be a few feet in one or two dimensions.

I recommend separate machines.