As mentioned before I'd go with a machine that has the largest hoop I think I'd ever need. Started out with a Viking +1 and within days I traded it in for their Designer 1 back in 2000. I fell in love with embroidery and knew I'd want to do larger designs. I still have my Designer 1 but use it mostly for regular sewing as I love all the features it has. I picked up a Babylock as I didn't need any special programs to get the designs from my computer to my machine as I did with the Viking. Now Embird makes it possible to transfer designs for most all machines excluding Bernina I think.
I'd go for a machine that has at least a 5 x 7 if not larger, needs no special programs and buy from a shop with good customer service and hopefully lessons to show you how to get the most out of your machine.