The primary focus of the video editing workstation is going to be your computer. When you are going to build your own video editing computer workstation, you have to select the machine that is going to meet the needs of your post-production work. In general, you have a choice between Apple's Mac computers or Windows-based PCs, unless you know a lot about working with Linux.
Though many of the industry standard video editing programs are available on Windows-based PCs, the Final Cut Studio is only available on Mac computers. Final Cut Pro - and its associated programs - is quickly becoming the top used video editing package and you will find that running a computer that cannot employ it will be unnecessarily limiting your options. Since After Effects, Photoshop, Avid, and Pro Tools can run on both types of computers, there is no reason to go with a Windows computer when you are looking to build your own video editing computer workstation.
Even if you select a Mac computer, you may not get everything you need to run your programs. In general, large video editing projects will take a lot out of your processor. Color grading programs can take a lot of power, not to mention motion graphics and animation programs. All of these are going to need to be in play in a full video editing computer workstation, and so you need a computer that will bring enough power for this. On top of this, you will also need a computer that will support media inputs and outside storage media. This means you will want to have a computer that can read and write DVDs and Blu-Ray discs, have USB and FireWire ports in a relatively large number, and a storage size that is considerable.
To meet all of these needs you may want to go with the Mac Pro system where you will purchase a monitor separately. This is going to end up costing you quite a bit more than an iMac, so if you want to build your own video editing computer workstation at this level then you will want to know that you need these kind of features.