Hi Carlos,
If you are in the UK you should join the Lancia Thema & Dedra Consortium
www.lancia-tdc.co.uk Members of teh consortium can get a kit with Cam Belt, Balancer Belt and tensioner bearings for around £150. We also have dimantled more than a dozen Themas and still have some complete cars, so almost every part is obtainable.
The difficult parts to obtain these days are the rear cross members. Check under the car to see if this part looks excessively rusty.
As mentioned before, you need to listen for noisey tensioner bearings. If after revving the engine there is a rattle from the left of the engine bay (as you look in from the front) then that usually indicates excessive bearing wear.
The handbrake can often be a problem with these cars. Quite often the cable stretches with age and only one caliper is activated by the handbrake. If left unattended the handbrake lever on the caliper will get sticky and seize. Reconditioned calipers are still easy to get for £80-£90 from most Motor Factors. Changing the belt is a 3 hour job.
The electrics on these cars are good and only trivial stuff tends to fail. Bodywork is usually good, but cars that have lived in coastal areas or salty road environments may have some structural rust at the rear jacking points so poke and prod with a screw driver in this area. In general these cars are much less rusty than their SAAB 9000 counterparts and they handle much better too.