I'd like a secondary system for myself too, I'd go with
Ryzen 1st gen - exceptional value at these prices, 1600 would be my pick
good b350 board, msi pro carbon most likely, never had a msi board, would like to try
16 gigs of fast b-die memory (trident z looks awesome)
rx580 8gb or vega 56 sapphire - great value cards from a superb AIB partner,plenty of vram and good perfromers all round
m.2 ssd - no need for much storage on seconadry rig, go for speed
mx34vq, would be great for racing and sports games, beautiful design and great specs at a very acceptable price
phanteks p400 case - very fond of looks
140mm clc, nzxt x42 preferably - why not, I saved on the cpu, I should spend it somewhere else
any super flower psu -this brand name is just too hilarious to even consider any other psu brand, I had mine ordered abroad,paid a premium and waited for two weeks just cause I found the name amusing
sadly, I'm never satisfied with my primary rig, so all the money goes there.