“Mk12 build” why even say that?
Agree. As a builder of Mk12 rifles, not sure why you even are buying an SPR profiled barrel. The SPR profile is made to accept Ops, Inc. / Allen Engineering brake and collar to fit the AEM5 suppressor. If you are not wanting to shoot a Mk12, why try to change it.
But, one thing I would say, is that Centurion's roots are in the Mk12, as Monty was a Mk12 SEAL sniper. What he produces, in terms of rifles, barrels, receivers, rails, are his commercial version of what he would have liked to have shot. So, if you follow that logic, you can stay with Centurion and buy their parts and be very happy, especially as you mentioned that suppression was not a priority.
I am still wondering why you picked a Mk12 and then want to change everything... We did a test on major SPR barrels a few years back, and most that advertised as Mk12 or SPR profile failed the test and would not adequately support the AEM5 suppressor. Not sure why. Allen's profile is in the public domain, so it is not hard to do it right, but the majority of barrels we tested failed the test. SO, It is a good thing that you are not looking to support a suppressor on the barrel you have chosen, as they did not meet the test when we tested, although, that might have changed.
If you like 16", then go for it. The Mod Holland Mk12 is a 16" barrel. The 18" rifle barrels have historically had early port erosion issues, whereas the 20" do not. In a visit a few years back with Frank White and Compass Lake, he showed me how much better the Douglass barrels held up to port erosion, compared to other barrels. But, as best we both can tell, Douglass is using the same 416R stainless steel, so not sure as to why. The port erosion issue I think has more to do with the shorter dwell time on an 18" barrel in rifle gas.
In theory, you will have the same issue with shorter dwell time with 16" barrel in intermediate length, but I have no empirical evidence to support that. The shorter dwell time is a somewhat newer preference for those shooting suppressed, and those in the know, like Bobby West at CGS suppressors, will swear by the most reduced dwell as possible when shooting suppressed.
So, you have a good barrel coming, and now you need to build around that. The rail choice will not affect accuracy. Get a rail that works for you. We tend to like Geissele, Midwest, LaRue, Centurion, to name a few. All of these will come in MLOK or railed, or a combination of both. Receivers are important, as is the BCG.
We tend to work with military correct receivers, so we have a few that are great, but I suppose I would expand on that list to include Vltor, which builds a ton of good stuff (just not military grade). You can also look at a Noveske Gen3 or Gen 4 receiver and rail. Great combo. Expensive. For BCG, we really like Microbest.
As a rifle builder, we have settled on Microbest as our premium in-house brand. Most of the better BCGs branded by other companies are Micobest. LMT and Geissele would be outliers as superior BCGs, but probably you are not going to see the difference in performance unless you run 15,000 rounds through your gun, and your barrel will be toast before then.