Does it already have a rail on top? If not, you'll want to get a mount for it; I recommend a HK-style claw mount.
Second--you're going to want a fairly compact scope. Long scopes, or ones with huger objective lenses, can get in the way of the cocking handle.
And finally--you'll want high rings in order to clear the rear sight.
I've got an ACOG on my MSG91; but that's a bit more than you want to spend. You may want to look at Trijicon Accupoints.
Or, see if you can find one of the surplus Hensoldt scopes from the G3SG's that were available a few years ago; they come with the claw mount, and are usually under $500.