the 500n is a sort of 80s 90s family holiday slr, with only 3 AF points and limited overrides. Having said that its still quite a capable performer and i've had £900 of L glass on mine before i bought an EOS30, and then a EOS3. Its metering / exposure was always pretty good to my eye. Takes all Canon EF lenses right up to current day, lots of choice but also some real wallet busters !! Some of the cheaper plastic bodied lenses aren't half bad once you know their limitations, especially when ou can pick them up for £15-30 a piece.
The OM10 is earlier but built for the same sort of market - lots of cheap lenses on ebay, so plenty of opportunity. Probably the cheapest way to accumulate lots of lenses.
You really need to assess the condition of the cameras and their operation first ***.
No idea on the Praktica i'm afraid