Common problems:
Vac pipes - replace them all if they haven't already been done - £15 'ish, dead easy job
viewtopic.php?f=6&t=39514Crank sensor - £40
MAF - second hand £45 ish
The engines themselves are pretty robust to be honest. The weak link is the injection pump as previously mentioned. Many have been replaced in their lifetime. Don't ever run it out of diesel or attempt to start it after a filter change without properly bleeding the system.
In general they are a good bus. A read through the 2.2 section should throw light on many of the issues.
EDIT: Another essential bit of kit is a code reader. The ELM 327 works well with the Dti and and at under 20 notes is good value.