Im thinking of buying the season 1 DVD but i've heard that the episodes are incomplete. How many minutes are missing from each episode in the season 1 DVD set?

2-3 minutes per episode.

Why were they reduced then?

They weren't technically Reduced, The first season release was a release of the sydnacatied episodes (the version you see on locals or back on tbs/oxygen) are are 1-2 minutes shorter than the original broadcasts. They only did it because Carsey Werner would not supply the original copies of the episodes, however this has been fixed for the remainder of the episodes, If your still not happy with the first season you may have to deal with it as it is very very unlikely they will ever rerelease the first season...