As previously said, it does make more sense from a resale point of view to buy in January but if you ask the dealer nicely, he will almost certainly agree to you leaving the car at his premises until Jan 2.
Its a very good point about buying in December, but as far as registering a car in January, Poles buy second hand cars on the year of manufacture, not the year of registration. In regards to buying a car in Germany, I have done this and you must remember the import tax and related costs,
This is what I wrote on another thread:
I imported a car from Germany about 14 months ago, my experience was very straightforward.
Be aware of the following:
Tax on full purchase price under 2 Litre engine in PL 4%
Tax on full purchase price above 2 Litre engine in PL 14%
If you buy a car that is younger than 6 months it is classed as a new car in PL and taxed as a new car.
If you buy a car that has less than 10,000 KLM on the clock it is classed as a new car in PL and taxed as a new car.
Any car that is imported to Poland, for insurance purposes is worth 10% less than the same car bought in Poland.
I found my car on this website mobile.de
When I take into consideration all costs after purchase price, tax and expenses of going to Germany, the same car in Poland through the dealer would have cost me about 50-60,000 PLN more than I paid.
In Germany there are company's,and a group of people who are locked into buying a new car every 6 -12 months, they buy at a deep discount, run the car and then try to sell at a profit, before the new car is ready. These are the people to target if you are going to buy a car in Germany, because the transaction has to go through the dealer, so you send the money to pay off the contract to the dealer, not a private bank account.