Prebuilts are generally worse since they cost premium and if you are going to swap the GPU then better assemble one on your own. But to answer your question, yes it is future proof and will do a good job for the next 2-3 years.
Yeah, it will run all games. Maybe you will have to drop to medium settings but it will do the job. Better however would be assembling a pc on your own. Cheaper and more efficient.
Well you don't have to build "build" it yourself. You can just go to a pc part store, give them the list of the parts and tell them to assemble it. They take something like $10 fee to assemble the whole thing and it will still be like $200 cheaper.
I am wondering why you picked the i7. I guess without knowing a budget that's great but you're really not going to see a boost in gaming from the hyper threading. i5 would be fine if you wish to spend the savings elsewhere. Definitely upgrade the graphics and SSD prices are dropping like flies with deals for 250gb Samsungs at $70 or less. If you use rebates, you can get the price way down from $1400.