First off, you don't need the dvds to make suggestions, episode guides can help there (tv.com seems to be a good one place stop).
Secondly, you may be interested to know that the price on the dvd sets for DS9 and TNG have bee dropping. They've gone down to ~$55 per season, DS9 is up to season five, with TNG a season or two ahead, and a new season dropping every few months or so. I've seen them at Walz Mart and Amazon, haven't seen them at all at Best Buy or Ciruit City, and haven't looked elsewhere.
If you're o.k. getting things used, you should be able to find them even cheaper.
