My 1.3 (736kg payload) bought new in 2006 is still going strong. It has had me one up 400L. Me and Dan the Man 2 up with 400L and a trailer with 200L for six months till I bought a new van. We kept it.
Now Dan the Man is on his own and running it for the last 4 years with a 400L. He was thinking of upgrading but like he says it's runnnng fine and so he saves a few hundred a month towards the new one.
Engines
1.3 is fine for one man and a 400/500 (new shape 750kg) NO CAM BELT it's a chain.
1.6 has the worst reputation of the three. Less well proven and reputed oil feed problems if not religious about oil and filter changes. May be OK now. Dunno. CAMBELT
2.0 is the JTD /HDi we all know and love from Peugeot and Citroen engines. Better if 1000kg payload.
the 1.6 is not the same as the citreon peugeot, it is a FPT (Fiat Alfa Lancia) engine. it is basically a downsized 1.9 found in the older Doblo's and many other Alfas and Fiat. the new 120bhp 1.6 replaces the old 2.0
the 2.0 16v multijet is no longer avalible as it is only euro 5, it is a gm based engine not Citreon Peugeot so get yours facts right, I have a Doblo with this engine, very rare there are not many about and power wise defo the best.
the 1.3 now has 95bhp so more power than the old earlier Doblos, most swb Doblos have the 1.3
whichever engine you get it doesn't really matter they are all good, buy new if you can or get one which has been well looked after which is rare with commercial vehicles