I might not know much about the M&P yet but I can help with this!
First, don't plan on saving money if you build your own. I build mine so that I can get the exact configuration that I want, the first time. Instead of buying one off the shelf and reconfiguring it I would rather build it the way I want from the start. If the configuration you want is readily available from S&W, Colt, LMT, Stag or RRA (to name a few) then buy a complete factory rifle.
Second, you can build one that is just as reliable and sometimes more reliable than a factory carbine if you know what you're doing and you use quality parts from quality sources. I've had factory carbines go TU in the first mags and I've built carbines that ran for 5K without a single issue. There are several things that I know do to each and every AR I own, whether I build it or buy it; add a HD extractor spring and Defender or Crane O-Ring, re-stake the gas key and add an H buffer and an ISMI buffer spring. These simple and cheap upgrades help ensure a carbine runs successfully.
That is, of course, after I've used quality barrels, uppers and bolt carrier groups. Hope this helps.