Well AMD are pretty good on motherboards- so you could get a good AM4 motherboard now and it should work with the upcoming 3000 series processors (just make sure it's listed as compatible from the web store when you get the board, many of the older boards will need an update installing to make it work, which is a problem if you don't have an older cpu on hand to get it started up in the first place).
An AMD X470 board would be a good option, or you could look at a new X570 which is also just about to come out (those are designed for the new cpu's so will definitely work, again though you will have to wait a bit).
Processor wise, if you want a decent all rounder the upcoming Ryzen 5 3600 looks like it should be a good choice (the 2600 / 2600X are already good and the new version should be faster all round and AMD has listed MSRP at $199).
I'd recommend investing in a good quality power supply. It doesn't have to have massive wattage (a good 600W unit should handle all of this easily), but you want a supply with good quality components and protections included. Toms do a nice guide on this:
https://www.tomshardware.com/reviews/best-psus,4229.html
You also want fast storage, so look for an NVME SSD for the primary drive (these are very fast and used to cost a large premium but aren't too bad these days) and maybe a large Sata SSD for additional storage if you need it.
Edit: Almost forgot, memory: it's a good idea to get fast memory to go with Ryzen. The new Ryzen 3000 parts support DDR4 3200 out of the box so that is the minimum speed I would look at. 16gb is generally enough for most things so a good 2 x 8gb set would be a good starting point- if you get a motherboard with 4 memory slots you can easily add another couple of modules in the future if needed.