DEF diesel engines

Hi, I was looking at a few used DPs and noticed one was listed as non- DEF engine. Can I get some opinions on the advantages or disadvantages of getting an older non-DEF engine ? I am only slightly aware of this DEF thing but assume it has to do with the ultra low sulpher fuel. Most of the older coaches seem to have lower HP and I am a little nervous about lack of power after reading many posts about 40 ft DPs with 300- 350 HP engines being under powered. Thanks, Ken