there are companies that specialise in the design and construction of mezzanine floors.
the main concerns would be
getting building regs and planning permission; as it's a business I think this would be mandatory with regard to insurances etc. the last thing you'd want is it collapsing on your charges!
following on from that making sure the thing was structurally stable; as a structural engineer I've come across one or two mezzanine failures in my time. this would include making sure it had an adequate support structure, whether it is built into existing walls or supported on lightweight columns.
As a way of creating extra space they are great, so no real downfalls there.
cost wise, it would depend on the square footage, what the support structure was like (ie whether any strengthening is required, what investigations need to be carried out etc)
I'd suggest contacting a local architect or engineer and geting them to work out the feasibilty of it for you. You'd need an architect for the building regs side of things and an engineer for the structural aspect.
hth