I had the pleasure of working in an Alfa dealership, 00-03. As above, cambelt at 80km, but make sure you use a shop with the correct tooling. Both V6 and TS engines have NO timing marks, the cams are locked in position for timing by CNC caps that fit over one of the lobe profiles, the pulleys with correctly tensioned belts are then locked to the shafts, and timing is set. Without these tools the cam timing will never be correct.
Also, unless the shop has an Alfa diagnostic tool (EXAMINER), they will struggle with the late cars, and if you have a Selespeed, forget it. Selespeeds need careful set up of clutch and shift gate calibration that is impossible without the Alfa diagnostics. Well set up they are a pleasure, poorly a nightmare, (dont expect well set up from new).
In sum, a good independent with correct kit wil care and do the job. A dealer will have the correct kit, and is unlikely to care, unless you're lucky (me :-)), but a well meaning independent without the kit may waste both your and their time on anything other than routine servicing.
Edit: In nearly four years I never saw a 156 come in on a transporter, 166 yes, 156 no. Plenty of stupid troubles, yes, but they didn't STOP.