The gas block on the S&W would need to be removed/replaced if you ended up wanting to change the handguard. Not a huge deal, but another step and more parts to replace. General consensus is for a first AR you should buy one fully assembled. I went the 80 percent receiver route and assembled everything myself on all of my builds because I was more interested in that aspect, I have other firearms I can go shoot if that was all I was interested in. Have built 1 LR-308 (AR10 is armalite pattern only), and a couple 556 builds for the gf and myself. It's pretty fun to build them but can cause some head scratching trying to diagnose issues. Most of the parts on mine were Aero (Ballistic Advantage for barrels, Aero owns them). The best time of year to buy parts/uppers/lowers/etc. is Black Friday, Primary Arms and a few other sites usually have them much cheaper than Brownells and especially Aero directly.
If you plan to build you'll need an armorer's wrench, or crow foot wrench depending on the barrel nut, torque wrench, punches and screwdrivers. Reaction rods are very nice to have as well, or action blocks like the Magpul Bev blocks. I'd also suggest a tool that will help you line up the gas block. A lot of issues come down to poor gas block alignment, feed ramps needing polished, and buffers/spring combos.