What seems to be the top things to change on this? Swap the barrel? Does anyone have any recommendations for when buying this upper and what spare parts I should have on hand?

I looking at putting red dot on it, and will be running it suppressed mostly.

There's nothing to change out of the box. I'd get some spares if you plan on doing beta mag dumps all the time. Gas tube, barrel, etc. Other than that just go run it.

It really depends on if you intend to set up the 11/15 as a lightweight carbine or a heavy, drum-fed, bipod mounted rifle. The IRM model is solid out of the box with its 10.5 barrel. Mine has run great on steel-cased Wolf Poly-Performance since day one. Magpul D60s work well as does the Armatac SAW drum, if you can find one.

The stock choice really matters most and changes the feel of it considerably. I’ve tried mine with a variety of them including a Lage prototype fixed stock, a Lage collapsible, a Lage K-folder and a custom ACR DMR Magpul PRS2 stock (which is for sale in the EE because I’ve gone back to a lightweight configuration). The Lage fixed stock and ACR DMR look good and offer a much better cheek weld than the other two.

Lage sells a M11/9 AR stock adapter as well but it looks a little Frankenstein-y with most Magpul, Vltor, LMT, BCM stocks.

Best of luck.