This is definitely NOT a good idea ... at least not economically. If you're planning to use UnRAID basic (free version), you're limited to 3 drives ... so 2 data plus parity means at best 400GB of capacity with 200GB drives. You can buy a NEW 500GB drive for ~ $50; refurbished units for ~ $35; and some used ones on e-bay for even less. If you want more than 3 drives, you'll need to buy a Plus or Pro license; and if you're doing that you'll almost certainly want to use higher capacity drives as well. For example, if you bought a Plus license and 7 200GB drives, you'd get 1.2TB of space. You could build a larger system than that by simply using the free version and 3 750GB drives. Ditto for Pro ... the economics simply don't add up.
... not to mention that newer, high capacity drives are MUCH faster than the older units.
i already have unRAID Plus license on my 3.5TB ver 4.7 system. It runs flawlessly (and I don't want to mess with it). BUT I want to dabble with a ver 5.x system on the cheap. I don't need any high capacity drives to run a ver 5.x system. using 3 120 (or even 80) GB drives is more than enough to test a ver 5.x install. these drives can be had real cheap on ebay.