If your passion is to build machinery, then go for it! If your passion is to build other things and need the machinery to do it (in other words, you really don't *want* to build machinery but *need* to so you can build other things), then I echo the other postings saying that there is a LOT of good iron out there which would probably serve you well.
You would be surprised what you can find in junk yards. In fact, it can bring tears to your eyes sometimes. If you were in the US, there is what was probably a beautiful Hardinge HLVH lathe on a government website right now. The only problem is that it is in Georgia (I think) and clearly was left out in the rain for the last 5 months. The thing is one giant rust bucket. In fact, the tray is full of water. Ironically, in this case it is not a typical DoD auction either because their "starting bid" is $1500, while most auctions start at $5 (from what I recall). But not all junk yards are like that. And sometimes you can get lucky with just CraigsList
I was casually looking at CraigsList once and saw a nice, but older, CNC bed mill. I contacted the guy and told him up front that I really was interested only in checking out its size in real life, but that I already had a mill. I was not someone who could justify getting rid of my other and buying his. Well, 6 months later the guy called me. He had a proposal. He didn't know the machine (but knew it worked because his father had bought it new) and wanted the space. If I paid for a rigger to move it out and then reposition two large machines, I could have it for free... with tooling. He even let me keep the machine there for almost 4 months while we figured logistics out and we each had vacations. In the end, I got a really nice machine for about $400 (and then spent another 1K doing stuff to it that I felt needed to be resolved... but that was my choice).
Now, those types of deals are rare enough that you can't wait for them, but there definitely are a lot of less sweet, but still very good, ways to purchase very good iron out there. And if you are willing to scrape ways, then that opens up a whole new avenue... And don't discount the cost of gears in a threading box either. They are expensive. On a machine you buy, however, you'll likely have all of that already.... unless you are planning on an electronic leadscrew, etc.
If you are truly interested in making your own machine because you love machinery (a very viable love, I might add... a hobby in its own right) then there are some excellent videos/sites out there which chronicle people's progress. Most of these are CNC bed mills, but not all of them. And some of the designs are really really good. I happen to love machinery and I have often felt that if I had room, I'd like to build a bed mill. But, I only have so many hours (and space).
In summary... It is all really about your motivations. If you *want* to build, then do a bit more searching on the internet. Otherwise, you are probably better off searching the back corners of used machinery dealers. They'll likely have some smaller manual machine that has been sitting there for years. Sure, they'll ask $4000 for it, but they very well may take $500.
