It really all comes down to what, specifically, you want at the end of the day.
For a 16-18", lightweight large-frame AR rifle setup like it seems you want it and capable of MOA or better accuracy, the factory options are fairly slim.
One excellent option that comes to mind off the bat is the Armalite AR-10T Carbine (16" SS match barrel threaded 5/8x24, Aluminum FF tube, 2-stage NM trigger, etc., etc. guaranteed for MOA accuracy, but all will out-perform their accuracy guarantees with good ammo and a competent shooter). The 10T Carbines as shipped from the factory weigh right about 8.75lbs with an empty mag and can generally be outfitted for <10lbs with a light optic/mount/etc. They can generally be found for ~$1600 or so and are one of the best bangs for the buck across the industry. The downside to the Armalite, unless you can find one of the extremely limited production "A" models that accept DPMS-pattern mags, is that you are limited to Armalite factory steel mags in 5, 10, 15, 20 and 25 round models or converting M1A mags. Not a big deal, but many people want DPMS-compatibility for mags because they want to run PMAGs. I have owned a couple AR-10T Carbines over the years in various configs as offered by Armalite (I still have one that I've tweaked a bit). They are boringly accurate rifles right out of the box that are sub-MOA capable with match ammo and quality reloads.
About the only "upgrade" you might want off the bat would be an adjustable gas block (~$100 for an SLR or Syrac...I prefer the SLR design) so that you can tune the rifle for both suppressed and non-suppressed fire as you stated you'd be running it, as well as any swaps for stock/grip/etc. you might deem necessary. The 10TC comes equipped with a basic A2 buttstock and A2 grip which helps with the weight, but they aren't always the best or most preferred options depending on your end game. Even still...with the basic AR-10TC and an adjustable block, you are only into the rifle for ~$1750 or less.
As for building what you want...it cuts both ways in terms of costs, etc. You can get a decent receiver set for ~$500 or so (depending on make/model/etc.). Tack on another ~$400 for a match barrel, ~$275 for a BCG, ~$200 for a trigger, ~$75 for a LPK, ~$100 for an adj gas block and you are ~$1550 give or take into a rifle without stock/hardware, grip, handguard/FF tube, muzzle device, etc. It can be done on the cheap, but to really get the most out of it in terms of accuracy and performance, you can easily get yourself into more money than a factory rifle will run you plus upgrades, but you'll have precisely what you want at the end of the day. It really comes down to what specifically you want to put into it, the options you desire, etc. as to whether the cost/benefit analysis comes out in your favor or not.
Finally, as to the custom built route...here you get to have your cake and eat it to, but you also have to PAY for it. A GAP-10 or JP will run you at least ~$3k (or more depending on options/configuration), but you'll have one of the finest semi-auto rifles made at present, made exactly how you want it, and you'll have a rifle capable of printing into the 1/2MOA range (give or take) depending on your abilities. A GAP-10, at least if you buy new from GAP, will cost you also in terms of wait time because you aren't getting into one anytime soon. They pop up on the Hide for sale from time to time, but by doing that, you may or may not get what you want and may have to settle for some other config which sort of defeats the point to going custom. JP can generally get rifles in their "Ready Rifle" program out the door in fairly short order, but again, will limit your options/specs somewhat, and their true customs for complete rifles will likely run you at least 2 months and likely 4-5 for completion (they have a schedule of production with time estimates on their site that is generally fairly accurate...wait will be dependent on what specifically you order, parts availability once your order queues up, etc.).
Anyway...just some food for thought.
