Motherboard wise it really doesn't matter, AS Rock offers alot of bang for the buck and good quality, same with MSI. ASUS is still considered the best and I've heard good things about Gigabyte. I would get a B450 board, mATX or ATX is up to your case.
For RAM Ryzen 2600 is less picky but you honestly cant go wrong with Corsair Vengeance LPX and GSkill Ripjaw is also good (and usually cheaper). I would check the motherboard manufacturers support page for each board and see the QVL list for RAM compatibility. That's not saying a RAM brand and model that isnt on it wont work but it's still a good reference. Whatever you choose buy the ram in a matched kit, so for 8GB get the 2x4GB or for 16GB 2x8GB. Youll want duel channel memory.
As far as your PSU, depending on how much is left in your budget, get a EVGA G2 or G3 550-650 watt or Seasonic FOcus Gold 550-650 watt. I you need a more budget friendly one the Corsair CX550-650 (Grey and Black text, not the one with green or builder model) or a Corsair CX550M-650M would be a good option. You could probably get away with a 450 watt but it wont leave much room if you get a beefier card down the line.