I've already built custom Zynq boards and they are fine for lower end compute, but the 1GB ram limit (imposed by the Zynq HPS memory controller) and only 32 bit DDR3 total width is kind of limiting, which is why I'm looking at other possibilities as well. Unfortunately the Altera SoCs are not nearly as easy to source, so I'm mostly limited to Xilinx stuff.
The Artix-7 is limited to a DDR3 PHY rate of 800MT/s, so I'm more gravitating towards the Kintex-7 which can do 1600MT/s (because of its "high performance" 1.8V/1.5V-only IO banks). The smallest Kintex-7 that is easy to get right now is the 160K LE XC7K160T-2FFG676I for just under $40, and I'm thinking about whether I should get a few for future experimentations. The Artix 7 with 100K LEs go for about $20. The effort to lay out >32 bit DDR3 isn't trivial, so I should pick only one family/package combination and stick with it.
I don't think it's worth it economically to build one or two because from my experience it takes at least two iterations and 3x wasted chips to get a design with lots of BGA working. The reasons being you can't test bodge fixes, and the soldering is always suspect so you end up assembling at least 2 of each design to troubleshoot any issues. But if there is a card/module design that will be reusable over and over, then it might make sense. I think the breakeven point with this project will be 5 units.