You’re going to have to pay to play with all of them, but the HK MR762A1 is the only rifle of the lot that is ready to go right out of the box. You can buy the HK and the only added expenses will be buying extra mags since they’re proprietary.
The B&T needs an added stock, which isn’t that difficult but given the $4k price tag, it’s just an extra $100-$400 depending on what you decide to slap on. There’s something to be said though about the compatibility of the platform with AR components like the buffer tube and trigger. Just more money you need to drop.
The SCAR is the worst rifle of the lot for out-of-the-box performance. Accuracy capability is there, but the rifle as a whole is very crude for precision work and you need to put in a ton of money in the rifle. The trigger is garbage, the stock is meh and the handguard is blah. When I was looking at seriously buying a SCAR 17, it was clearly obvious that I was going to have to dump an additional $1k into the rifle to upgrade it to the point where I was going to be happy with how it shot. I’ve shot both a stock 17 and an upgrade KDG/Geissele setup and the difference is tangible, and the added expense worth it for the upgrades over the stock rifle. That said, is it worth it to dump that money into a rifle that isn’t a dedicated precision platform? Meh, not really. At least not without replacing the barrel with a match barrel, and that’s even more expense on top of the whole deal. At that point you’re pushing well past SCAR 20 money, so what’s the point really?
If you want a hidden gem, the Sig Sauer 716 G2 DMR is a tack driver and gun that not a lot of people know about. Sort of a sleeper in the DMR world. My agency (large metro PD) tested one for our SASR program, where we were looking for a couple SASRs for urban quick response to complement our bolt guns. The G2 DMR was by far one of the best overall rifles we tested. We wound up selecting the Nordic Components NC10, but there were some mitigating factors to that selection, including the fact that NC and my agency happen to be in neighboring area codes. If we had not selected the NC10, the G2 DMR would have been our SASR.
Of the ones you selected, it’d come down to the B&T vs HK, just like what you mentioned. I think that the HK is a more complete package, but in the end the B&T might be more ideal for the compatibility with grips, trigger and mags. The mag thing could be a big deal, because HK mags are 3x as expensive as Magpul PMAG-LRs. I guess it just depends on how many mags you would want. The aftermarket trigger thing for the HK is where you really get hosed though. I guess at the end of the day it’d come down to what the accuracy performance will be. We know the MR762A1 can perform since it’s the foundation for the new M110 SASR, so I guess it just depends on whether the B&T can perform.
I would NOT recommend that you do anything with any legacy battle rifle (M1A, PTR91, FAL, etc). They’re antiquated, finicky, much harder to source parts for, and substantially harder to service/modify. Modern rifles are so much easier to service and reconfigure. Most modern rifles (if configured properly) out of the box can give 1MOA or better of consistent accuracy, while the legacy systems won’t get anywhere close to that either consistently or at all.