I wouldn’t put together a xeon based build for a first dl box unless you’ve built some machines before, - unless you find a workstation that is ridiculously cheap. Takes a bit (or a lot) more research and less people using them, plus xeon motherboards can be non ATX form factor and may not fit in a standard case.
I’m no expert but the MB you mention above looks decent - PCI Express lanes are listed by your CPU and will be a max of 24 for Z270 chipset, The motherboard you mention then can utilize these lanes in following config.: 1 GPU at x16 PCIe 3 mode or 2 GPU’s at x8 PCIe 3 mode.
Something worth doing is use pcpartpicker and look at DL builds at a reasonable price and copy the builds that have commonalities.
Use pcpartpicker to give you an indication of power consumption of the build then add 100W for safety, I’d err on the side of caution and have some spare power capacity to draw up - I think I read somewhere PSU’s are more efficient at <80% load but not 100% sure on this. I just saw above you found a 650W PSU for a great price.
ECC ram may be trickier to find a MB that allows this, if you are doing research/commercial products and need <<1% RAM failure rate and really want reproducability of your experiments/stability then go for it - most home Deep Learning builds I’ve seen (me included) just use standard consumer RAM which is easier to come by and I think cheaper.