The motherboard has native m.2 slots and support, it'll only disable a Sata port but that's no different than using a Sata ssd. The 960 pro is an NVMe drive, basically twice the performance of a Sata 3 ssd. The pcie I'm not sure exactly how that'll work on that board, but for testing, nothing has really been able to touch Samsung except in a few criteria.
I'm an advocate of new ram. Over the years I've seen too many people have issues with different kits, but there's also many more who have no issues at all. It's a gamble at best, but honestly, I'd go with a new full kit and ebay the old stuff since it's still viable ram.
I'm used to hdds. After 30 years of dealing with them I'm used to their quirks for sure. They are like old reliable transportation, not fancy, not spectacular performance, but cheap and reliable. If you choose to run nothing but SSDs, I'd still go the 960 pro for boot and important apps, but I'd go for something like a large crucial MX300 Sata 3 for storage where performance really isn't necessary. Even a 2Gb Seagate is relatively inexpensive, so if you need lots of storage this would be a better option than a pure ssd system, especially cost wise.
Not a fan of sli. You only really get great performance from a select few games that have excellent support. The majority have mediocre support and there's more than a few that either have no support or even negative results. You get better playability on a single gpu with sli disabled. So for me, a single more powerful card is better. It might not have the fps of sli but it also has no negatives and in some cases is far better than sli. It's a better overall score if you take the whole picture into consideration, not just those select few games where sli is king. So for me, a gtx1080 would not only be preferable, but cheaper than sli all around. Pound for pound, a 650w psu is cheaper than the same model at 850w usually. Personal favorite would be the Asus ROG Strix, but the MSI Gaming X is also a strong gpu with excellent cooling.
There's always a better option, just depends on who you are. If you look at the recent 'best builds' contest, in unlimited budget there are pc's in excess of $100,000. But they are workstation/uber servers with a sidereal bonus of gaming rig. So exact what's better depends on you, more than the components. Personally, I don't like the H440 much, I've no use for the triple fan in front and it's not a quiet or very good airflow case, but it looks cool,even with the green glow and the additional expense of the Razer name. But others think it's the best case since the invention of pre-sliced bread. Opinions of what's better will vary somewhat.