So this build is better than any Dell at this price? I would like to have i7 though if I build it myself.
The custom build one is faster and last longer?
Yes it can, a 3rd generation socket 1155 i7 which would be a 3770 or 3770k.
Those numbers are specific i7 model numbers. For instance, 4770k = 4th gen Haswell Core i7 for socket 1150 based motherboards. 3770k outlined above. 4930k and 4820 are 3rd gen Core i7 models for socket 2011 motherboards.
The K means it can be overclocked and a K CPU is wasted on a motherboard like