I find some website: krakowdirect.com/auschwitz_tour/,
Do you know it's a good idea?
When I went back in 2008,and as I didn't speak a word of Polish at the time, I opted for an Auschwitz tour.
I went with this company: cracowtours.pl/auschwitz-birkenau-memorial-and-museum,tour,1.html
I couldn't fault them at all. They pick up and drop off at various hotels in Kraków.
Although it's possible to get a bus to the camp, and entrance is free, I was glad I went with a tour company as the guide was so informative and I learned so much more than if I'd chosen to go it alone.
So yes, I think going with a tour company is a good idea. There are many tour companies operating, so your only problem will be choosing which one.
If you are on a budget, the tour company you are asking about charges 50 PLN more than the one I went with. The price for one person is 44 Euros, which is about 184 PLN. Cracowtours charges 135 PLN.
I'm sure whichever company you choose will be fine though, and many offer a reduction if you visit more than one place.
I opted to go to the Wieliczka salt mines with the same company. Definitely worth visiting if you have enough time.