here's the cheapest set of FAST ram I could see on brazil amazon
https://www.amazon.com.br/Memoria-Corsair-Vengeance-3000Mh-CMR16GX4M2C3000C15/dp/B06XRFNWHK/ref=sr_1_9?ie=UTF8&qid=1541657512&sr=8-9&keywords=DDR4+16GB
As for the motherboard, I just remembered, A320 is 1st generation Ryzen, and for it to work with 2nd gen, you first need a first gen chip to install so you can get the BIOS updated.
But I don't know what places you have to chooses from for online retailers.
Basically, see what you can find for the cheapest dual channel memory kit, whether it be 16GB (2x8) or 8GB (2x4) and 3000MHz or higher in speed.
But that's only if your current RAM doesn't work (which honestly, it should work just fine)
As for motherboard, whichever is available in your budget that's cheaper which would be a B450 or an X470 motherboard.
Otherwise, you will have to settle for a first generation Ryzen CPU and then you can more easily find B350 or X370 motherboards for less.
But if your current RAM is slower than 2666MHz at the absolute very least then it will hurt the performance of the Ryzen CPU. Because Ryzen performance is directly linked with RAM performance and faster RAM means faster CPU even at stock clock speeds. Having faster memory will make it perform better at stock speeds than with slower RAM at stock speeds.