You say very fast HDD, how fast is that?
As for saving up, I got a different way of doing it that works just as well.
Instead of saving, I buy the parts one or two at a time depending on what I can afford, then put it all together when I got everything.
Well they are normally 10k RPM HDD with fat load of cache. Not going to be as fast as a SSD, but they will be alot quicker than a normal HDD. How much faster? No idea lol, I haven't looked up fast HDD in a long while, sure someone here knows some good fast HDD.
Assuming you have something really cheap, it should be at least twice as fast as a good guess.
Oh, one more thing, SSD and fast HDD, are in general small in size, around 100gb area (Some HDD ones are around 200gb but they get pricey). There are bigger SSD but cost way too much.
If your going to get a new rig, I would just buy a nice large HDD, for storage, 1TB+ and then eventually when you got the cash, buy a SSD, and use that for your OS and main programs that you use.