Decent room to work? How much 'work' do you do after you built it?
Ummmm.... I'm talking about when he builds it? You know, sliding the MB in, routing power and sata cables, hooking up the front panel, etc...
Why do you need tons of space for this? It is something you do once. He isn't going to build it with his feet.
So when you add a new HDD, you change your whole PC?
Or never add a second graphics card?
Or more RAM?
Why would I add a new HDD? I'd either build with plenty of HDD capacity to start with or get a separate external rack for my HDDs if I had some weird storage fetish and it could be hidden in a desk cupboard.
Why would I add a second graphics card? Twice the noise, twice the power consumption, and I have to get the same one I'm trying to upgrade. I'm buying outdated crap. Its time for a new PC if you need another graphics card later cos you will want faster ram and a better processor to accompany it.
Why would I want to add more RAM? I built the PC for a purpose. Ram is dirt cheap. I'm going to put twice as much as I need in there anyway. My next realistic step will be faster ram. For that I will likely need a new mobo, ergo new PC.
Build a PC Dave and see. You don't end up upgrading them. Between f'd up upgrade paths and requirements of several components at once, you build a PC for now, and start saving for the next one unless you are on the bleeding edge and buying the latest everything as soon as it comes out because you have a money printing machine in your bedroom.