at op
No.1 most expensive part will be the GPUs.  So take your time choosing.  I would go with either RX 5600XT or 5700(XT) from AMD
or 3060TI / 3080 from the nVidia side.
No.2 is PSU.  Buy 2x 750/ 850Watt PSUs instead of one single 1600W PSU.   These things are expensive.

There are a lot of dedicated mining mobos to choose from.  Or get an ASUS Prime mobo that has 6 pcie slots.
You can get Z270-Ps second hand.
Then there is a whole industry doing mining frames for GPU rigs.  You could get a Veddha or a Banggood.  Or design or build your own.   Your choice.
You need risers.  Any from version 006S and up are good.
Generally, prices have gone up a lot due to the all coin new moon season.  Even 2nd hand parts.