The HK91 is a roller-delayed blowback. It's not a locked breech system. At least not in the strictest sense of the term.
HKs have a variety of options for scope mounts. Unfortunately, none are cheap and some very hard to come by. Most likely, you're looking at the HK mount itself or the ARMS knockoff. Both sit high and you'll have to buy a new stock with a cheekpiece for a proper cheekweld. Of those two, I'd prefer the HK despite it's painful price. ARMS advertises a return to zero, but I've had experience with three different ARMS mounts and it ain't so. The ASRM (no, not a typo) systems were best of all, but no longer available. If you can find the weaver base one, grab it. The Warne base mount is good, but you're stuck with using Warne rings.
The HK's barrel has no interface with it's bipod. It's not really free floated, but HK accuracy is usually very good. The limiting factor is the gun's traditionally horrid trigger pull. Figure a Williams trigger job or a PSG1 trigger mechanism as a must have.
Lastly, a full up HK91 weighs a ton. Even stripped bare, you're looking at about 9 1/2 lbs. With scope, mount, MSG90 stock, and a full magazine...about 12 1/2 lbs. Add the heavy forend and light bipod and you've got another 2 lbs.
I like my SR9T (HK91 variant) a great deal. It's perfectly reliable, very accurate, and just plain fun to shoot. It's no fun to carry around though. It'd probably be a good stand hunting rifle, but I've got much more appropriate guns for that, at a fraction of the cost.