You could figure out which sockets you will need the most and buy those first. Then grow your socket set as $$ allows. I've seen many guys build there 3/4" drive sets four or five sockets at a time. Although best option would be to find a lightly used set at a good deal.
Seach Lista on Ebay. There is a distributer who has tons listed on there. I hope BMWpower will chime in. He has a thread with lots of pictures and info. I have been thinking about getting some Lista since they are local to me. About 1/2 hour from my home/shop.
How old is it? What condition and how much? I own a few Millers and they have never let me down. Replaced the trigger on the mig gun a couple times. That was probably due to me being dropping it or tossing it back onto the cart. Awesome machine and very easy to get parts for if ever needed.
Not...