Executive summary: I'd say $700 cash for a 9mm Mark III is fair and not to be picky about grips or sights if a good deal comes along.
1. What do you mean by "Belgium-made"? Do you mean one that wasn't cast in Portugal? Those will be older and more expensive, plus they will not be Mark III and thus shouldn't be used with +P ammo very much.
2. I paid $600 for my used Hi-Power, although it came with a few upgraded parts. One store near me has a new Browning Hi-Power in .40 that could probably be had for $700 cash. That strikes me as an okay price for a new BHP.
3. Don't get too hung up on the sights and grips: those can be upgraded later if you come on a good deal for an otherwise-good pistol.
4. Go for the Mark III if at all possible. I think Browning only imported those in the finish you want, but Mark III is much preferable to the earlier models unless you're looking for an Ingliss or Nazi WWII pistol, which it sounds as though you're not.