Looking at this as a base: http://www.servethehome.com/supermicro-x10sdv-tln4f-review-platform/ How does this look future proofing wise. The other similar model is without the dual 10Gb and DDR3. For portability, power, noise and future proofing this seems like a great base?
One thing that I have experienced with ESXI is that hardware compatibility can suddenly drop with new versions. In my case the network ports didn't work moving from one version to another. I would think the same would apply to other peripherals like raid cards or anything pci-e you would like to pass through. I don't see CPU requirements for running VMs changing anytime soon, so if you are happy with that now then it should be OK. Really the only other real bottle neck that I have experienced is with the amount of available RAM. Obviously the more you have the greater your concurrently running VM capacity will be. To sum up, build for compatibility with the latest ESXI version, any subsequent version is a crap shoot and make sure you have loads of RAM.
Is your focus on learning about ESXi and vcenter or more to do with the VM's you want to run? If you just want to learn about ESXi and won't be running a ton of windows VMs, I would actually flip to the exact opposite end of the spectrum and build 2 boxes based around Celeron J1900s and a 3rd with an i5 of your choice. You'd use the i5 (stuffed to the max 32GB) as your main ESXi host, one of the Celeron j1900 boxes would run Vcenter and something like openfiler which would be used to store VM's such that you could use vmotion to try transferring a live VM to the 3rd Celeron j1900 box. If you're more interested in running a ton of windows VM's and don't care about vcenter, then 1 box will suffice. If you stuff an i5/i7 box with 32gb of ram and have a ton of SSD's, you'll be able to run a surprising amount of VM's in there before you start to see any performance hit. Naturally you need something 13x6, 2011 or the likes to go above 32GB. You could also wait for skylake which is rumored to support 64GB; 16GB un-buffered modules are super rare/expensive so you'd probably need to go with a xeon variant.
Personally I'd get something like a HP ML350 G6. It'd be a lot cheaper in the end. I've got one running dual CPU. 144GB RAM & 8 SAS drives. Runs happily with nested ESXi boxes as well (so I have 3 ESXi boxes & the vcentre server all in one box)
The number of tinkering I have done with White box ESXi builds, sometimes it just doesnt appear to be worth the effort. Next time round, I'll just be getting some microservers and going that route. (Non production learning / playing environment)
