Jack,
If you look around, I have seen reasonably modern AMD boards that take reasonable priced AMD processors that often go for really low cost. Don't be afraid of second hand AMD processors, as long as they are in good nick, they will work just fine.
The X99 boards are hard to get and really expensive and while they run the E5 family of Xeons, you only see the advantage when you use a high core count. I have 2 12 core E5-2690s and while they really hoot with multi-thread work, they are nothing exciting for general purpose computing and the combined package of board and processor is too expensive.