The old game is no longer sold, AFAIK. Dragon Knight Saga is already the updated and heavily modified gold edition including the add-on. 20 bucks for 100 hours doesn't sound like that much.

The demo is no longer identical to the game. DKS changed quite a bit. Complete rebalancing, updated graphics, etc. The combat is now more like hack&slash in the lower difficulty levels. The devs did this on purpose to reach a certain rythm. So play on hard if you want a challenge.

The add-on is even pure hack&slash on top of a great (if compact) RPG, but very well done. You know when you'll need your weapons and when you're safe.