Car ownership

We have bought a house in Italy, but can't buy a car?!?

we are Uk citizens with a retirement visa and a receipt for our first PdS application.

do we have to wait 5 years?

No.  You will be claiming residence as you are living here with a visa, so you can buy a car.  You just need your residence first.

Is residence 5 PdS annual cycles?

Whether they give you a PdS for 1 or 2 years seems a bit arbitrary, but your residence will/may have an expiry date which is the same as your PdS, and will therefore have to be renewed when you renew your PdS.   After 5 years you will acquire permanent residence status.