Law /
Car registration in Poland (bought a car from Holland on a temporary license) [25]
I have imported multiple vehicles from Netherlands to Poland (im from Netherlands).
Taxation is as you say, i believe 2.2% under 2liter and 18.8% over that (might be off by 0.something% as i think they change every now and then)
For me it went okay with the value on the sales contract most of the times, and once they did not agree on the value (even though i never messed with the contracts, but it was lower as the car had slight rear damage).
Anyways, since the car was a hybrid with very low emission/engine size the difference in taxation they wanted was less than 200zl, not worth the bother to fight it.
Then there's about 260zl of costs to get your license plates and stuff.
I've all but 1 time, used a company that did all the work for me. In total it cost me 350zl for the company and about 150zl for getting the necessary documents officially translated.
I highly recommend using one of these companies unless you want to go super-cheap about it, but as you're not Polish im gonna guess your Polish is not good enough to get through the official instances. They do not speak English, nor are they in my experience lenient towards helping you in English in any way.
The one time i didnt use the company but a Polish speaking friend, it took much, much more time and hassle.