Well....my thoughts are this: build the best computer you can afford.
Also, look at things initially that are harder to upgrade, not cost effective, I.E. motherboard and CPU.
You can build a very nice system to handle everything you need pretty cheaply, if you scavenge parts from other computers.
For example: use an old case and power supply, used DVD drive, and put a new MB, CPU, Ram, and SSD in it for cheap...then worry about buying storage drives for your video. Craigslist is packed with great stuff to reuse for really good prices. Old gaming desktops usually have primo parts, and can be bought for next to nothing.