Looks like I am going to go against the grain here but this is my experience.
Many of the previous comments are correct but you need to decide on a machine based upon your needs. And a budget of $500 is not going to get you much if anything.
I bought the HF44991 which is a Seig x3 clone similar to the grizzly you mentioned. I bought this mill for a specific purpose keeping in mind that I did not have the power or space available for a larger unit and especially a knee mill. And I never intended to mill anything very large. The carry over to car support would be in the way of various brackets and mounting tabs all of which an X3 clone are very capable of making. Since having my mill I have put it to a lot more use than expected and so far I have not run into anything I could not make or modify within reason.
I did add a few upgrades to mine which combined exceeded the cost of the mill itself but I found that the upgrades proved to be worth the expense since they increased reliability, repeatability and precision.
If you have the room and power definitely go for the biggest you can afford but do not dismiss the versatility of the mini mills.
Pics of my setup;