I recently acquired an old Canon EOS 750QD SLR with a CANON ZOOM LENS EF (35-70mm, 1:3.5-4.5). If I was to purchase a Canon Digital SLR (at the budget end of the SLR spectrum) could I use the lens from my old camera in the D-SLR?
There is no reason at all why an old Canon lens will not work with a Canon digital camera provided you can get a suitable connector for your EF. The lens will work with any SLR camera not just Canon. If you go on eBay you will find lens converters at many prices so there is really no need to throw away a perfectly good lens just because you go digital.
Yes, but you must remember that you'll get a magnified image on any SLR you buy that doesn't have a full frame sensor.
The budget end won't have full frame. To get the equivalent in 35mm terms, you need to multiply the focal length by 1.6.
So for your lens it will give the same field of view as a 56-112mm fitted to 35mm. In other words you lose the wide angle nature at the 35mm end but gain a bit of telephoto!
Also note if you buy any new Canon EFS lenses, they are designed to fit Canon's digital SLR's (apart from full frame sensor SLR's) and cannot be used with your old EOS 750.
The s stands for short focus, what actually happens is the rear of the lens protrudes further into the camera body. This would foul the mirror movement on an old EOS, and in fact the s lenses are made in such a way that they are impossible to attach to an old body.