For RDP you really don't need much - even a 20 dollar t420 (everything soldered and practically a zero client) will take care of it. The question is why I picked the t520 (1.8L) - because with a 6w TDP CPU it could arguably run with a smaller PSU and take up less power while doing so - the t530 (which is the next step up) will give you the same firepower as the GX217GA on the base model 620 (2L), and it'll be slight bit smaller (1.4L)
That being said, I would not buy any t520/t620/t620 plus machines unless I have a very specific use for them, considering that there are t640s at the 100 USD level on eBay already, and those have very good performance for their price. As
@Patrick would have put it on his m75q Gen2 video, you can have one new machine do the work of 4 of those old ones - they'll take up less space, require less power and you won't have to source DDR3
SODIMM or MiniPCIe NICs to upgrade/wire them up.
Also, hold off on the Wyse 5070 for now. Wait until the HP t540 hits the general availability and see if Wyse is announcing a replacement for the 5070 series - I got a feeling that they will be announcing something for Q2.