I had a German made HK 91 back in the 80s. While robust and functional I didn't like it much, and got rid of it when I got an M1A, a rifle I liked much better and one that is much, much more reloader friendly.
Can't help with the collector market today, sorry. I can say that generally what collectors want most, and will pay the most for are the most pristine, uncommon variants. And a rare variation in worn condition may be worth more to a collector than a NIB condition common version.
I'd look first at the stocks. The "slim" forend is less common than the fatter one, and the green stocks less common than the black ones.
Generally aftermarket additions lower the value to a collector. Additional factory items may not, and may add value, depending on the collector, of course.
Good luck.