Custom build - local supplier
I agree with @liamk887 regarding building your own - tends to be the best value for money.
Apple
Their hardware tends to be a little underpowered compared to PC options and limited choice.
Workflow
Based on your description, optimising your workflow may be as much about screen real estate as computer hardware. For general office work, moving from a ~18" to a 30" monitor increases productivity by roughly 30%.
Consider 2/3 monitor setups.
As so much of our workflows are single threaded, an overclocked CPU can be a noticeable advantage - consider some of these options if you have a large budget.