Surface 3 - eMMC storage, Atom processor, 2Gb-4Gb RAM, 10.8 inch screen, 3-point kickstand. Avoid.
Surface Pro 3 - Core i3/i5/i7 (Haswell), SSD, 4gb-16Gb RAM, 12.3 inch, full travel kickstand.
Surface Pro 4 - Core m3/i5/i7 (Skylake), SSD, 4gb-16Gb RAM, 12.3 inch. More kickstand travel than Pro 3.
Surface Pro (2017) - Core m3/i5 (fanless)/i7 (Kaby Lake), SSD, 4gb-16Gb RAM, 12.3 inch. Kickstand goes almost fully flat.
In short, you'll probably still get good performance from the Pro 3 but the Pro 4 and Pro 2017 are excellent iterative improvements. The Pro 4 can suffer from screen flicker issues, so inspect it fully before you buy.
This is what I was looking for. Thank you very much!