If its any help.
I've done this job twice now as a DIY on a 2005 147 16v Jtdm.
The first time was about 1 yr after we got the car @4 yrs old and 43K and the most recent at 9 yrs old and 92K; so I think its safe to conclude - change the belt at 4 and not 5 yrs.
I to was told that tappets and rockers were required the 2nd time around- I didn't bother with both just the rockers - Its recommended by the manufacturer as an arse covering exercise as you have 2 metal surfaces that wear together (the ball on the tappet and the seat on the rocker) Most likely all the tappets will be OK - take it apart and have a look - it should be obvious if any tappets are broken or worn.
1. You don't normally need to replace the tappets.
2. You do need to make sure none of the existing rockers has split to the extent that some of the roller bearings have spilled out into the engine - as happened on the second occasion and got washed into the sump!
3. Note the replacement rockers may have different spring clips to join then to the tappet - this doesn't matter.
4. Make sure you mark the injectors as they need to go back into the same position, as they are coded/calibrated to that cylinder.
5. Do fit a new rocker cover gasket and 4 new injector seals
6. Get a cheap timing tool kit of Ebay or similar.
7. A set of cheap 'crows foot' spanners for torquing up the high pressure fuel pipes is useful - Note you may need to grind some metal off the outside of the larger size spanner to make it fit between the CYL_4 pipe nut and rocker cover casting- your see what I mean when you get there.
Note I got my rockers from Vauxhall (its the same Fiat engine used in the 1.9 CTDi) as they were £6.15 each and not £18 each that Alfa wanted; its worth noting you could be offered 1 of 2 types of rocker arms - The type is dependent on the dia of the valve stem.... it looks like later engines had smaller diameter valve stems and hence the later rockers are narrower where they fit over the stems and hence wont fit on the earlier (wider) valves. I suspect the earlier, wider type rocker could be used in both applications as there is only about 0.3 mm difference between the two types.
I think the earlier type rocker is significantly cheaper than the later type to!
Euro Car Parts also lists all the parts required, including the rockers, even if the rockers are more expensive than Vauxhall
The first time the belt went I just replaced the 8 damaged rockers, the 2nd time I replaced all 16 rockers.
16 rockers, 1 cover gasket, 4 injector seals from Vauxhall £130 (Sep-2014)
Timing belt kit approx £100 Auto factor
Water pump approx £40 Auto factor
Timing tool approx £20 Ebay
Crows foot spanner set approx £15 Ebay/Machine mart
If you need any more info just ask
All the best