@Johno - No direct experience here, but as a general point of caution, if you haven't already, I would keep tabs on the latest firmware releases over in the UI forums (
1.9.3 as of now), and read at least several pages of feedback to gage quality and stability for yourself. It's been well over a year since release and Ubiquiti still seems to be having a rough time getting the platform stable enough to call it even "mostly reliable" for a home setup (let alone a business). That of course hasn't stopped most from just rolling the dice, but it appears a portion of those who did are now reverting to other gateway solutions, UniFi or otherwise.
Also, on the wifi performance front, I would also make sure you plan to spec at least a UDM plus one or two more APs alongside, as the UDM has the equivalent of one FlexHD AP built in (with
sub-500mW power output), and on its own won't give you anywhere near the total capacity or range of your two Asus units (even in AiMesh, which lowers total WLAN capacity). I presume you've already planned for that, but if not, I would do so, including any cabling work required to provide additional ethernet port(s) for the additional AP(s).
TL;DR - In case you're not aware, business-grade APs (the UDM included) are purposely designed with
lower power-output per AP (to serve higher client densities), so to create equivalent performance across a given area versus over-amplified consumer boxes, you'll need a tighter density and higher overall numbers of APs. In a typical house, this may only mean one more AP, but still, you want to account for that upfront, so you're not left wondering why your new, fancy setup seems "worse" for overall wifi performance.
As far as potentially making a better product suggestion for you, are you running anything out of the ordinary with your Asus gear, like OpenVPN, certain Merlin scripts, etc, that you'd like to have in the new solution? Or are you just simply looking or more reliable distributed wireless? If you can detail any of that, it would help to confirm whether a UDM or something else would be a best-fit for your needs.