I don't think this is a great idea if you want the end product to look real good.
To get a better look for cheaper, I would search craigslist for someone that is ripping out their old hardwood floors. They should sand up to looking brand new. I recently redid an attic hardwood floor- about 800 sq ft. And the reclaimed wood only cost me $75!
Okay, so I've done this. I'm not sure it is recommended for everyone.
Mine turned out fine... but... recycled floors are A LOT of work.
* There may be nails to remove. I probably had 10 lbs or more. And they were cut flush with the back of the board, making removal more difficult than if they were poking out the back.
* the edges may not be straight. Mine had quite a bit of EXTREMELY HARD finish on the mating surfaces. You may have to process each board to scrape this off. I ran both sides of mine through a table saw to clean them up. This was very time consuming.
* You'd be surprised just how different the widths of the boards might be. They may all be 2 1/2 inch planks... but they may have come from different manufacturers. There will be odd variance of 32nds of inches here and there. And some will have shrunk more than others... The table saw will help here, but you may end up with some imperfections here and there. (I decided to just embrace them... and make it look like the old floor with character that it is.)
* Now, unless you are good at putting together puzzles... you are not going to put the floor together the way it came apart. This means that the height will be different.. It has likely been sanded before. Different areas of the floor will be slightly different heights. You need it flat. It is going to take a huge amount of sanding with VERY aggressive sandpaper. (The rental shops around here didn't carry sandpaper that was aggressive enough.)
* And because the wood is old, it is likely to be REALLY HARD. It gets harder over time. This means all the cutting and sanding is going to be really hard work and will take several blades and a pile of sandpaper.
I'm not saying I wish I hadn't done it. I love a good project. But I'm not sure I'd do it again.