I would get dual channel. A 2x4GB kit won't break your budget, and still leaves two slots free for another 2x4 (or 2x8) later.
I consider 120GB too small in a general-use PC; in fact I'm using over half my 512GB SSD in my main PC. I do have a 120GB drive in my "lab assistant" PC, but it has few programs on it since its primary jobs are to serve as a target system for LAN throughput tests, and I can grab screenshots and record results on it while I'm running my tests, or even kill a few zombies. For general use though, I probably wouldn't be "suffering" with a 250GB-256GB drive, but it is convenient having more.
The new Corsair CXM PSUs (NOT the older CX) have reviewed well, are modular, S6/S7 compatible, and are perhaps another $5-$10. It was excellent in its day, but that particular Seasonic has been left behind by advancing technology.
Once again, the original build had no bad choices; a 2x RAM kit is a change to which anyone I know would agree (a 1x kit might make sense on a board with only 2 RAM DIMMs, with a plan to get the second later), but the rest was fine.