Dedicated rig or machine for everyday use?
If the latter, just buy a machine that serves your everyday purpose - then your overheads are near zero and all farming is profit even at low margins.
If dedicated, see the posts above for ideas. Personally I think low power / always on is good, and keep your costs down. It is doubtful whether pro farming will be profitable, so spending a lot is risky IMO.
You don't need lots of CPU power, but a balanced solution. Responsiveness and throughput are important. Trouble is, we can't know in advance where the sweet spots will be, and they will change as the network changes.