I think I'm going to go with a Maximus IMPACT mITX board, an i5-4690K, and a 500GB SSD, possibly M2.
I'll re-use my SG-650, GTX970, Define Mini case, Xigmatek Gaia, 8GB of RAM, and a pair of 1TB drives in RAID1.
The first upgrade will likely be to 2x8GB of RAM.