get a few cheap 750ti's and google ccminer , since you are new i would stick to something simple.
(there is a cryptoblog that has some good simple info on setup for you )
i would not try both amd and nvidia ... stick with one and learn it.
you should be able to build a rig with 3 or 4 - 750ti cards in them each.
dont spend alot of money on MB, cpu and case... just try to get mb's with lots of pci-e slots to start out... you can learn about risers later
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if you can find one that has 5 pci-e slots or more...
try to leave a slot open between cards... the spacing can save you a lot of trouble with heat issues ...
a few 80mm case fans and some hot glue also goes a long way to keeping the air moving over the cards
just make sure to buy a watt-meter so you do not blow fuses or start any fires!!!
3 x 750ti @ 60 watts each = 180watts plus another 100watts to be safe for the rest of the computer.
and you should never go more then 80% of PSU so buy at least a 500watt gold PSU
i always got 750 or 850 ... then i had room for more cards later
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good luck ... but just look at it as fun ... then you might even make a few $'s
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