In my opinion, right now we have two "best" options, if you can't find 3080s
RX 5700 XT and non XT - easier to find, and probably with a good price - hashrate 53 Mhs without Bios mod and 56 to 58Mhs with Bios Mod - consuming 140w to 150w
3060ti - hard to find at MSRP but easier than 3080s, reasonably price with 60Mhs, easy to tweak, consumes 130w and will have a better resale in the future
I would go on 3060ti, but if it's hard to find, buy 2 or 3 5700s and look for 3060tis to complete your rig
The motherboard will depend on how many cards you want, there's good options from Asus (in my experience, Asus is the best for mining with desktop motherboards), or you can buy mining boards if you want just a rig (not a PC to use daily)
PSUs will depends on how many cards too, but if you want 4 or 5 cards, you can buy 1200w GOLD PSUs, or you can use 2 PSUs, and you can buy 2 x 850w for example (you have to check price and availability to see which one worths)
Nicehash is good enough to use, don't worry
In the past I used to mine directly the coins and sell in the exchange, but today I prefer to use nicehash to save time and to get Bitcoins
Nicehash supports how many GPUs you have
A heads up when you go to bigger rigs and need two psus be sure to know how to wire your rigs. Or you will short shit out.
there are some deals for 1200watt psus if you look carefully but you may find that they prefer to run six cards better than 7.