If you want interchangeable lenses then you are looking at either of the two Leica mounts, screw mount or M bayonet. A screw mount system will be cheaper and be forward compatible with the M bayonet through adapters. The cheapest decent cameras for screw mount lenses would be the Canon RFs. If you just want to get your feet wet you could pick up a Canon 7 and a Russian 50mm (supposedly not entirely compatible) like an I-50, Jupiter 8 or I-61l/d. All are nice lenses and pretty cheap, like maybe $40. Together the camera and the lens would set you back around or a little over $200. You can move up from there perhaps to a better lens and then on to a Leica if you find that you like using RFs.