I like where you're going with this but you could take it a lot further with little expense. Amazon and MicroCenter have a huge stock of refurbished HP mini PCs with i5s, 16gb of ram and an M2 slot for about $350. Throw in a small m2 ssd in each (~$50) and keep your UPS, that's about $1700 you have a decent cluster for an efficient stack like phoenix.