If its an old EZ series Canon flash (example 300EZ) its not compatible with digital cameras. It can be used in manual flash if the flash unit supports it. It uses an older form of TTL called A-TTL and its not compatible with digital cameras. It won't be able to provide automatic flash control on a digital camera it will fire at full output only. Now if the lens is a 3rd Party lens who knows if its compatible or not. Mechanically yes but electrically it may not be. If the lens is really the EF 75-300mm F/4-5.6 lens lineup its a very poor lens optically and has a slow AF motor and has no IS (Image Stabilization). The EF 35-80mm is film era lens and it too has a slow AF motor. All of Canon's EF lenses can be used natively on EOS film and DSLR cameras. I wouldn't suggest going with a DSLR camera. As Canon has discontinued its DSLR lineup including EF lenses. I would suggest looking into the EOS R Series mirrorless camera lineup. Who knows how long Canon will keep supporting DSLRs and EF lenses. More and more lenses are going away along with DSLR cameras.
-Demetrius
Bodies: EOS 5D Mark IV
Lenses: EF Holy Trinity, EF 85mm F/1.8 USM
Speedlites: 420EX, 470EX-AI, 550EX & 600EX II-RT
