Personally I think a wooden base is fine, it's all in the details!
If you want grey for concrete, I would go with some kind of press/fiberboard/chipboard, and skim it with some kind of plaster, with slab lines, etc. That would look better than just painted.
As for a wood-wood base, almost any wood can work with the right stain, but something hard is better than soft pine. I just go to the hardware store, the bigger ones with timber sections. When I was in the US I would get oak, because it's very hard and is gorgeous. Here in Australia they have Meranti, which I guess is some kind of exotic wood. (Technically it covers 4 types of hardwood). It's a darkish pinky/brown (depends on specific subtype), and not bad but not too beautiful, but hard. Stain that and it looks gorgeous!
For my Type VIIC I used two types of stain and blended, so it looked alive with character and not like a piece of stained wood. I'm doing the same for my Gato (did the base early, not sure why!), and I used 2 different stains to the other so it doesn't match the VIIC - not sure why i feel they shouldn't match! Anyway, the 2 or 3 stain effect is much nicer than just one. And if you get a darkish hardwood, just clear varnish/urethane warms it.
