As the Piper says, a cambelt service should really include the timing belt, aux belt, tensioners, idlers and the water pump for it to have had the full works done.
The JTD doesn't have a variator, that's part of the VVT system on the twin spark petrol motors, so you can ignore that.
The cambelt advice is also incorrect - cambelts are recommended as 3 yearly for petrol engines only, diesel engines the update service manuals list the cambelt as a 72k service item and note that it should be changed every 4 years for heavy duty use (cold climates, town use, long periods of idling) or 5 years otherwise - so if you stick to every 4 years for cambelt you will be okay.
Alfas do depreciate at a fair rate, I don't think its too bad but that depends on your perspective. My advice would be not to think about too much, unless you're planning on taking finance out. Any car will depreciate once you've driven it.
The GT and 147 are virtually the same car in a different dress - the 156 / 147 / GT have a vast amount of common elements across the vehciles - you'll notice the interior on the '47 and GT are near identical for dash etc.
The common rail diesel engines were actually developed by Fiat (having sold the rights to the common rail tech to BOSCH when Fiat ran out of money a little bit) and started out life in the 156 2.4JTD motor and was developed from there.
The common rail diesel engines are considered one of the best out there for its time and the 16v JTD M-Jet has a great balance of performance, power and economy. My 156 JTD M-Jet used to do 50mpg or more easily.
You won't find too many issues with the motor, the only thing really to look for is it being sluggish. Below 2.5k RPM points to EGR issue, above tends to be MAF issue. When you test drive make sure you do lots of 2nd and 3rd gear low RPM foot hard down accelerations to check for this and ensure the turbo works fine.
They work on a variable geometry turbo so you want to make sure it boosts properly across the rev range and doesn't throw any warning lights / limp modes (which it will if there is an issue).
Other than that biggest weakness on the platform is front suspension - check for squeaks and rattles / sloppy handling.
A flashing odometer means canbus errors too, but they're fairly uncommon.
2008 is the toward the end of the production run for 147s so it should be a well sorted motor by that time in its life. The engine will run on for years and years so long as the body is well bolted together around it. The clutch on my 156 JTD M-Jet went around 85k so check in history to see if it has been done, I know clutches depend on use but it could be coming up time.
Final word from me - Look for the higher spec Lusso, Collezione, TI etc models and consider going for maybe a slightly older vehicle with higher spec. Alfas always come with good spec, so buying a poverty spec motor now will harm you on resale later as higher spec should be easy enough to come across. May check out the Parkers guide I think that has some good spec info.
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso