Aug 29
Can I buy any of the Canon/Canon compatible lenses available for either/both Full Frame as well as the CMOS Canon DSLR and use it on my Canon EOS300 Film Camera?
Alternatively can I buy the Rebel Xsi DSLR and use my old Canon (film SLR) lenses on it?
On your film camera you can use all EF lenses. You can not use any of the EFs lenses. The EFs lenses are designed for cameras with APS-C sized cameras like your Rebel XSi. The 18-55mm kit lens for your XSi will not work on your film camera.
All of the EF lenses will work on either camera. Your field of view will be different though. The XSi due to its smaller sensor will give you more reach on the telephoto end. For example a 100mm lens will perform on your XSi as a 160mm lens would on your film camera. You will lose out on the wide angle end though. This is why the kit lens for the XSi is an 18-55mm lens instead of the 28-80mm lens that was the kit lens with film cameras.
For third party lenses look for lenses that do not say digital only or anything similar.