How do I buy rims online and know that they are the right size/lug pattern etc.

What is your question?

I have a 2004 ford freestar and I need to replace 1 rim. My local dealer told me that a new steel wheel from ford is $325. I've seen rims advertised at as low as $75 but how can I be sure it will fit my vehicle? Also, do I need to have tire presssure sensors installed in the tire/rim. If not what will happen to the low pressure indicator on my dash?