AFAIK, the 'heat shields' refer to the stock plastic handguards. IIRC, Colt added double heat shields to their plastic handguards since the shorter carbine length gas system tended to get hotter than the rifle length M16A2. Thus, if you're going with a rail, the heat shields are not a consideration (I could be wrong, but if you're worried about heat build up, when shooting a lot, just wear some gloves).
Re: Fixed Sight. Depends on the optic you want. With a 1x optic like Eotech or Aimpoint, depending on which model and mount, the BUIS is usually out of the way. For example, with a 1/3 cowitness, with the exception of the aimpoint micro series, the optic will usually sit higher up and the BUIS won't get in the way. If you need to utilize the BUIS right away, all you'd need to do is drop your sight picture down in to the BUIS.
I am planning on mounting an Aimpoint micro on a Larue660HK on my DDM4V1. This will give me absolute cowitness. I find that absolute cowitness with a fixed rear BUIS tends to get too 'busy' for me, so I opted to swap out the BUIS for a Magpul MBUS, which folds down and pops up. Also, the Aimpoint micro series has a smaller window, so 1/3rd cowitness with irons will give you a partially obscured sight picture, should you use irons. Hence, my decision to go with absolute.
Pw mm, user: mm
If you want a zoom optic, then you'll have to replace the BUIS with a folding BUIS (or forgo BUIS altogether, which some people do). Maybe there's some optic out there I don't know about, but due to eye relief most people will mount their zoom optic over their folding BUIS, like so:
Short Answer: Rear BUIS's cost $55 (magpul MBUS), so you can also swap that out easily if you don't like the included BUIS, or want to use a zoom optic.