Well, yes and no to many of the points you raise.
Yes, you can get a machine without spending 'too much' money, but you pointed out the drawback yourself, the shelf-life is going to be shorter. Yes, you can save money by building it yourself but I've never done anything like that and starting to learn how with $600, $700, or $800 worth of parts doesn't seem like it'll save me money. I'll mess something up, it won't quite work right etc. I'll have to put in lots of time troubleshooting etc. And yes, it'll save me money as compared to buying a new console, but I'm really only interested in Skyrim. I don't like other games, just don't (though I play Halo with a buddy at his house).
The real challenge isn't building a cheap machine, the real challenge is figuring out how many mods you want to run and how much machine you'll need to run them. There's really no guideline for figuring this out. There are so many mods that nobody is going to say 'you need a machine of spec X to run the following list of mods...' It's kind of trial and errror. I'm pretty sure I'll want to run lots of texture mods, NPC mods, upgrades to towns, home mods etc. I'd be better off with a beefy machine build by a vendor. I'd spend more time playing, less time fixing and troubleshooting. Just gotta accept the price-tag versus the time it takes to master the learning curve. Who knows, I'm getting older, maybe I won't find another job for a long time and I'll end up doing it from scratch any way.
Thanks for sharing your thoughts. I appreciate the enthusiasm of the PC gamers!