The best gpu to mine are rx 470/480/570/580...because of the electricity usage...but if u have free elec then I think the best choise are gtx 1070ti or r9 because of higher hashu...but they are more expensive...so u have to calculate what is good for u...don't mine btc...mine eth, etc and other coin's and convert it to btc if u want to have btc
Search on google/youtube...lot of articles and videos about how to get first rig...