Some report the most common problem were Glow Plugs for these engines. Do a vin check and see the history of the specific truck. Most glow plugs were recalled I believe. Otherwise the LBZ was a great engine. You'll want to know if the previous owner tuned it or anything like that obviously. Every truck is different so just make sure to cover the basics. If anything odd stands out have a mechanic check it out also.

As far as mods-yes you'll want to mod any duramax. Most basic is EFI live by atptrucks (Idaho rob)