welcome to the forums and to crazy crypto section
although I welcome the enthusiasm, right now may not be the best time to start building, hardware is hard to get or way overpriced, much better time will be when things cool off and some not so serious miners start flooding the market, it will happen, trust me, it always does, just may take 6-12 months
if you do want it, forget the radeon7, great card, but also very touchy and prone to failure, since you wont get it new, its a high risk investment (at its current price, tis def an investment)
look for the new nvidia 3xxx series, if you can find it somehow, not the 3060 12gb, thats cut down, the 3060ti, 3070 or 3080 are your best bets
also nvidia are far easier to setup, amd can take more fiddling
look for a good platinum or at least gold psu, seasonic or corsair or evga for example (1-1.2kw to have room)
motherboard doesnt have to be mining edition, any ol z270 or h270 with 6 pcie will do perfectly fine, the asus h270-plus can take 8 cards
but like stomp mentioned, tis best to find some gpus first, make sure you have at least 2 before going for the rest, oh you'll need misc crap like risers and frames later, you can get most here:
https://www.aaawave.com/coin-mining-tools/
as for places to buy .. well my fellow cryptonians here may be of better help since most are in the US, and im in EU where there are no places to buy right now