Re: PTR MSG 91
The overall length of the scope, combined with the size of the bell, is the biggest difficulty in mounting a scope to any HK91/G3 type clone rifle, including your MSG-91. The longer the scope, and larger the bell, the more difficult it can make proper operation of the cocking lever. The total length of the scope that you can get away with will depend on things like eye relief, how far forward you have to mount the scope, etc., but I generally tell people that 13"-13.5" or less is the best way to go. If you stick with a smaller bell on the scope, you can go as long as you like...just realize that the typical HK "slap" of the cocking lever is not going to work and you'll have to maneuver around the scope somewhat to charge the rifle, etc.
I can tell you that I have an MSG-90 clone setup with an old Mk4 M3 3.5-10x40 that works very well for me (for your budget, you may be able to find a good, used one, but there are plenty of other options). The other scopes I'd consider looking at that will work well on your rifle on your budget would be:
SWFA SS 3-9x42 (as well as the fixed 10x models)
Vortex Viper PST 2.5-10x44
Bushnell Elite 6500 2.5-16x42
There are plenty of other, highly suitable scopes out there depending on your overall budget, etc., but that's a start anyway. As for a previous recommendation on the Weaver Tactical 3.5-15x50...that scope is fairly long for the HK at around 13.5" which when combined with the larger bell may cause you issues.