As mentioned .. make a list of must haves and the nice but not required features/functions.... it is so easy to get caught up in the bells and whistles once you are in the shop..
Do ask what feet come with the machine. Then make a list of the feet you will add and the price of each. Use this when you get to a short list of machines to really get to a apples comparision of price.
I know I am tied to one of my machines because the cost of replacing all the feet would be quite substantial.