As any newly acquired used vehicule, you have to review every system on it, unless you know the past owner and maintenance.
Harmonic balancer and crank belt pulley are a good preventive maintenance to do. I suggest you use a good synthetic engine oil (in 5W40 viscosity preferred) and some fuel additives in every tank (FPPF total power or Stanadyne performance formula) to improve low temperature reliability and behavior (your engine - and you - will like it).
Richard Lajoie
2001 GMC Savana 3500 SLE 6.5 TD - 15 passenger (the way to go with 7 children)
3" to 3.5" to 4" custom exhaust, 3.73 diff.
Amsoil 5W30 HDD, synthetic media oil filter
Amsoil air filter
1987 Suburban 6.2 test vehicule