I think you're going to run into surface finish problems if you build an aluminum-bodied surface grinder. Cast iron is much more massive, and has far better vibration-dampening properties. I think an aluminum unit would sing like the dickens, and you'll see it in the surface finish.
I am all for doing rather than talking, though, so if you want to give it a go, I say do it!
The only other thing I'd add is to be careful. Make sure your (wheel) downfeed is precise and repeatable. You do NOT want to tell the thing to feed down 0.0005" and have it drop the wheel a few thou instead. Very bad things can happen. A thousandth or two of an inch on a lightly constructed surface grinder is a heavy cut.
Running a surface grinder is an art. There are many variables: work hardness, wheel composition and hardness, wheel size, wheel RPM, infeed rate, traverse rate, wheel finish (how fast or slow you've run your truing diamond across it). Plus the variables that are tougher to control: spindle bearing type and condition, condition of table ways, motor vibration. All of these will affect surface finish and accuracy.