I have to echo what the other posts said...building the cab can be more than half the fun, but only if you are interested in that sort of thing. When I started, I amost gave up in the first week because the rotating control panel I wanted to build seemed WAY beyond my abilities. I stuck it out and am almost done with the cabinet, with the rotating panel.
If you want instant gratification (like I did) build a "Frankenstein" CP first. You don't have to worry about graphics, or front ends or any thing else to start out and you can get it done in a weekend. It will also give you ideas about what kind of control panel you want and wehre to go next. I bought a sheet of 3/4" MDF and a 1 1/8" spade drill. I then bought a few buttons, a couple of joysticks and an I-pac. I was up and running pretty quickly, and then got a trackball and a spinner. The frankenstein control panel let me have the instant gratification of playing the games with "real" arcade controls and I got many ideas for the actual CP. Building the cabinet started last August and as I said I am finishing most of the painting just this week.
The best part of all this is that I am re-using all of the controls from the Frankenstein CP. Since I've slowly been acquiring my components, I've been able to keep their purchases under the radar of my wife! It can get pretty expensive with all the screws, bolts, hinges, etc.
Getting the control panel set to your liking and having someone else build the cab is a great way to go. $250 dollars compared to the amount of time on the learning curve is a great tradeoff, if you don't feel like getting the experience of building the cab yourself. I know I spent at least $250 on errors!
Lastly, there is no such thing as a purist, since all of these cabinets are amalgamations of different pieces/parts and only the real dedicated, exact replicas are "pure". The best part of this hobby is that you get out what ever you want. Some people just want to play games, some want to learn woodworking and some don't care about either one, they just want their friends to say "Wow! You you have a what in your family room?!"
Good luck, and don't forget to keep everyone posted on how you are doing!