Presumably you are talking about this one: http://www.apolloduck.co.uk/image.ph...157574&image=1
That looks like a reasonably tidy boat for the money; the engine is probably quite old (but if it runs well who cares).
I've never been on a Bombard 500 - but at a metre longer than a SR4 it has some advantage, even if it doesn't have the famous reputation the SR does. I think for the same money you'd be looking at a pretty scruffy SR.
Does it have power trim and tilt (a feature that makes little difference to the cash value of an old boat, but makes a big difference to the ease of use).
The obvious criticism is it has a bench seat which lots of people don't like compared to jockey seats. On the other hand they can be better for use with young kids.