The Sig is a VERY sweet rifle to shoot... the recoil impulse is VERY different from an AR15; almost "pleasant" in a way...
I have one of the old-school 556 rifles. SN puts it at about the 3,000th 556 made, old enough to still have Swiss internals but new enough to not have the finish & rail problems the first couple thousand had. I love it and I will never sell it, although I'm tempted to upgrade to a new style lower to get the folding stock.
I've already decided my next gun purchase will be a 556 SBR that I intend to convert to 300 BLK to get me a great back-pack/briefcase/take it anywhere you want to type of gun.
Parts commonality is worth considering but because you already have an AR15, I wouldn't be concerned about it. The likelihood of that ever coming into play is very, very unlikely. And if it does, you have a back-up gun... The more likely scenario is that if you buy the DDM4 and SHTF never comes, you will regret having 2 AR15 style rifles if you really wanted the Sig rifle in the first place. So consider the liklihood of what this rifle will actually be used for (most likely it will be used for normal range blasting and maybe varmint hunting etc) into your purchase.
I say if you want the Sig, don't lets the parts commonality of the DDM4 sway you. Just get the Sig.
There is no need to upgrade the lower if you want a folding stock. I went with a side-folding stock from ACE. Easy to install and cost me around $160 to do it. And you are right about the trigger pull on SIGs, they are an extreme pleasure to shoot.
There are more and more aftermarket options out there every month. You can change out most of the parts yourself, it's just that on some parts Sig won't sell them to you without sending the gun to them. Its stupid, and it's why I hate Sig, but I really like my 556 rifle. Best factory trigger on any battle-style rifle hands-down.I had a 556 and traded it for a 516, and then eventually the 516 for an AR platform. I loved shooting the 556, but missed the modularity/personalization you can get with an AR platform. The 556 was a dream to shoot though. The piston cut down on muzzle rise noticeably, and made for quick follow up shots. The only beef I had with it and the 516 was that if I wanted to do most of anything it had to go to Sig for them to install it (handguards, etc).