I'm on a low budget (£300-£400) on my gaming PC so I'm starting out with an APU for now and looking to add a graphics card in the future. Is the MSI B450M PRO-M2 a good motherboard for the Ryzen 2200g?
I think a 3600(X) or 2600(X) would do fine; especially the 3600(X) because Zen 2 is so much more efficient. But an 8 core is going to be limited with overclocking really not recommended. And a 3900X will probably not achieve near it's potential even in full stock configuration.
RUSerious: Tomahawk's are nothing like this board; Tomahawk's have probably the best VRM design in any budget AM4 board. It can handle even a 3900X overclocked.