I have done this in the past!.
As a poverty-stricken first time buyer, I've painted (and stenciled) concrete floors, wooden floors, mdf type, & chipboard. Least successful was the chipboard, more due to the finish of the wood than anything else.
The stenciled concrete floors looked surprisingly well; but it did take time to do.
Unibond (or similar PVA) sealer is essential as untreated wood will just soak up paint like a sponge.
Paint shops will try and sell you undercoat, I'm personally unconvinced that specialist undercoats are needed.
I have a big tub of cheap 'ceiling' paint (from Colortrend) which I have used as undercoat on just about everything I've painted and it's worked a treat.
Satinwood or eggshell finish from Colortrend will work very well on both wood and concrete.
I don't know that I would see paint as a long term finish for your floors though.
Garage floor would be a different story and I'd use the more specialist paints for that (colour range is limited, but that doesn't matter much for the garage).