Ryzen 3000 series are supported by both older X470 (and B450,etc..) and newer X570 chipsets. But this is mostly useful to people who already have a board and don't want to upgrade it, for new builders, the small price difference is well offset in favor of going with newer X570:
- PCI 4.0 is not useful at the moment, but it doesn't hurt to have it esp. since you mention you would like to get few years out of this build. The cost between comparable motherboards is under 100 euros, I find that surcharge to be worth it. For Threadripper TRX40, the same surcharge is three time as high.
- Air cooling is definitely capable just fine for 3950X, the "watercooling suggested" is pure marketing bait. It might be made to make sure people don't use low capacity air coolers, but something the size of Noctua NH-D15 or NH-U12a is not on par, but quite often better than most 240/280 or even 360mm AIO water coolers. Esp. in sizeable mid-tower cases with good airflow, there is no difference.
Most board differ on hardware level and feature set. The HW is mostly about power cascade/VRMs, how well it supports overclocking. Feature set is mostly about how much money you like throwing into trash bin :- ). Almost all ATX-sized board support dual-gpu setup. Very few support quad layout, if that's something you would consider in future.
There is 'Tier' list of quality for all available X570 boards,
https://docs.google.com/spreadsheets/d/1wmsTYK9Z3-jUX5LGRoFnsZYZiW1pfiDZnKCjaXyzd1o/htmlview#gid=2112472504 You can compare everything here if you have enough time.
For my build, I personally went with Asus ROG Strix-
E (the E being important, as the F & I version have inferior VRM for almost same price), because it's almost identical to much more expensive Asus ROG Crosshair while being significantly cheaper in Europe. In US they cost almost the same for some reason though. 250 +/- Euro budget is what I find reasonable for X570 board, above that you're paying mostly for unnecessary fancyness.
Now just quickly to memory and I am out. To get the most out of Ryzen 3000 workstation, you should opt for 32GB DIMMs for memory to give you option of 4x32=128GB either right away (I suggest doing so) or in future. No choice of actual memory on market though, just Corsair sets. In January, there might come more.
