What documents should I get from the seller.
The most important is to have a proof of legal ownership of the car, for example sale and purchase agreement. It's obvious. Next the vehicle registration card if the owner has it. But it's not necessary in regards of the classic cars - then you only write an statement about the lack of this card.
Is it odd that the seller has had the car for some time and not registered it?
So maybe he has an evidencing problem that he is a legal owner of this car? It's happens sometimes, that somebody finds an old vehicle in a barn or in some other deserted location.
The another thing is, that it's not so easy to register an old, classic car in Poland. The first step to register an historic car car in Poland is to get so called Biala Karta (white card). All major components of the car have to be original. The law states that at least 75% of all parts should be genuine.
The cost of implementation of such a card by an expert (in three copies - it's the rule), together with relevant photographs of the vehicle, will cost between 250 and 450 PLN respectively.
So think twice before you buy an unregistered classic car. It is not easy process to register such a vehicle in Poland.
Who should I contact to register it as a classic in Poland
If it's not registered car, and you are trying to register it basing on the "white card", then the situation is quite complex. With a request for vehicle registration and proof of ownership of the classic vehicle, you go to the vehicle inspection station. You need also to have an appropriate, completed application for registration of historic car. The form consists of 71 detailed technical questions about the vehicle plus attached photographic documentation. Link: isip.sejm.gov.pl/Download?id=WDU20130000337&type=2
If the car passes the technical examination, then you get the approval to entry into service.
The other required documents for the registration: owner ID, a decision on the inclusion of the vehicle into the monuments registry or a document confirming that the vehicle is in a central registry of the cultural goods.
Most of this entry is based on the online sources, so some translation may not be fully adequate.