There are a lot of free (demo) sample sets available - including some good sets that are both charming and demanding. You will find it very entertaining to try some of them.
On the other hand, I think in Hauptwerkland there is still some correlation between price and quality. A truly excellent, extremely charming AND very demanding instrument is the sample set from the Trost organ in Waltershausen. It has a lot of unique and colourful stops in a quite dry but pleasant acoustics. Any mistake will be punished!
Interestingly, it is probable that Bach has played this organ. See
http://www.organartmedia.com/en/engelfried/45.html for more details and
http://www.organartmedia.com/en/orders for more sample sets from Organ Art Media including pricing.