Halo: Reach will not feature any sort of microtransactions when it is released on PC (Steam, mind), according to the latest information from the developers themselves, 343.
Halo: Reach PC microtransactions were brought up recently by developers themselves, via a big AMA they held on Reddit, and not only have they revealed that there will be none of that stuff, but also that they'll be reworking the original release's progression system as well.
The original question which spurred the reveal asked if PC players will finally get the chance to experience the "amazing Ranking/Credit system", referencing Reach's intial progression feature.
"We're happy to say that Halo: Reach in MCC will in fact have the same level of "mix and match" customization found in the original games," said the dev in response. "As to how it's earned, we are exploring a new and deeper progression system that retains the feel of the old systems and blends them with newer models for game progression."
Better yet, the developer also mentioned that Halo Reach will have no type of monetised progression feature whatsoever, which means that this new unlocking system will be entirely merit and gameplay-based instead!
If this is the sort of stuff that you are excited about, perhaps you'd be interested in reading our take on why Halo 5, too, will release on PC in the future.