I commute to work everyday by bike and sometimes bike to the canal to fish. Don't worry, I never have and never will wear any lycra
As someone else said, don't expect it to get rid of your belly, but it will definitely make you fitter.
Brompton's are the best folders, but are really only suitable for the road or paved surface; on anything else you'd struggle. They're expensive but hold their value (2nd hand prices aren't much different from new).
For something that can take a bit more rough stuff, I'd recommend Dahon. They make a cracking range of folders, built to last and have a range that includes bikes for off-road/mixed use like the
ios d9 or
Cadenza which is like a proper folding hybrid.
As for gears, most full bikes these days have a stupid amount. Mine's got 27 I think, out of which I probably use about 6 or 7. Folding bikes usually have around 3 - 9 gears.
Hope that helps!