I have a 5-speed bike with a buckled back wheel. I have found one on the internet cheap, but it is a 6-speed wheel. Will it fit my bike? Will there be any adaptations needed to the bike to use this wheel?
Just because a wheel is buckled doesn't mean that it can't be fixed. You may want to take it to a bike shop. As for will the wheel fit, first note that you'll need 6 speed shifters unless you're using friction shifters.
As for the wheel actually fitting, you need to check the hub spacing -- for example, 5 speed rear wheels for road bikes normally use 120 mm spacing while 6 speed use 126 mm spacing. Measure your frame's spacing and buy accordingly. If the frame is steel, you can often cold-set it to take the wider wheel (which is what you'll likely need to do if you're going from 5 speed to 6 speed).