you do need to be a bit smarter when buying equipment for Linux (or Mac for that matter).
many devices come with Mac & Linux drivers
if you have to rely on finding an available driver look for slighgtly older products (ie not the most redcent devices) and look for products from big big companies (eg HP). that doesn't mena you cannot get a driver for modern equipment or from smaller companies.. it maybe harder.
many manufacturers equipment is compatible with previous generations. so for example more recnt HP pritners are braodly compatible withthe printer they have replaced, or other models in the current range... so if there isn't a precise driver for the printer you have try one from the rpevious range or one form the range beneath. (eg a 3500 series prnter may work fine with a 3000 series driver, or a 1600/2600 driver)
stick to buying stuff that is compatible with standards and youwill not go to far wrong
don't buy stuff that needs software ont he host computer (eg printer which use the computer from some work), stuff that interacts with the host computer eg multifucntion printers.
TWAIN scanners should be OK
Postscript printers should be OK
PCL printers should be OK
however in reality I found very few pieces of hardware that wouldn't work in Linux (they maynot be fully functional IE you cannot do everythign you coudl with the comparable windows drivers).
but quite often the support softwatre that used thise drivers was lacking or part finished