1.2 engine - slow but simple/cheap to repair
1.4 engine - getting there but still slow
1.3 engine - Fiats turbo diesel engine is surprisingly good, cam chain tensioners wear around 75k
1.9 engine - awful lucas epic system, requires new high pressure pump every 50-60k
1.9 JTD engine - definitely pick of the bunch, very reliable engine (used in many models and alfas) but remember to renew cam belt and water pump every 5 years. 50mpg.
Build wise the doblo is very good, no worse than any other manufacturer. A good choice.