I am not a hardware expert but I am in the process of getting a new system to edit HD. I went for Core i7 with lots of RAM. I had a look at the Dell system but it doesn't have USB 3 (although that is still a minor consideration) and didn't think that it matched some other systems.
Suggest that you speak to some of the suppliers (Chillblast, Arbico, Novatech, etc) and see what they recommend. This also gives you a good idea as to how helpful the supplier will be. I think that you will be pushed with your budget to get a system which is going to last for a while (having said that every system gets out of date fairly quickly).