Building a computer specifically to mine cyptocurrency is a big commitment, I spent about a week just doing research before I even consider putting a parts list together. When I started my build about 3 weeks ago, a 6 GPU mining rig cost about $2400-$2600, now if you're lucky enough to find video cards for $300-$350, your total cost would be about $3000.
If you still want to build a mining rig and you are committed in building one then you have to plan out everything.
Here are some of the items I considered when I was mapping my build and I'll try to give detail information about the items listed:
1.) Know the difference between the two GPUs (AMD vs Nvidia)
2.) Are you planning to dual (merge) mine or solo mine?
3.) Which coins are you going to mine? (Can it be dual mined?)
4.) How many GPUs do you plan to use?
5.) What is your cost of electricity?
6.) Which OS are you using? (Windows, Linux, or Ethos)
Here are some sites with some great information:
https://www.buriedone.com/
http://1stminingrig.com/
https://motherboard.vice.com/en_us/article/59zdjq/an-idiots-guide-to-building-an-ethereum-mining-rig
https://cryptosrus.com/ethereum-mining-rig/
Thanks for reading, let me know if you have any questions or suggestions.