Nobody uses lead anymore. Why? It's exceptionally difficult to do and is a big health hazard. The newer fillers are far better than lead, which had problems with paint sticking to it by the way if not properly cleaned and acid washed first (the process involves lots of beeswax on the wooden paddles used to push around the soft lead). Polyester fillers do not absorb water and do not shrink after curing which was one of the reasons plastic fillers got a bad rep years ago. Sanding scratches would show up days later under the fresh paint. The proper use of modern fillers is easy. Get rid of the rust. Acid wash the metal to leave a phosphate coating. Prime with epoxy primer. THEN apply the filler in thin coats and do not try to do more than fill small imperfections with it (metal working skills required) and sand. Prime again with a high build poyester primer. Sand. Apply a sealer. Paint. Seal new metal and welding from behind.
Quick-buck artists tend to skip several of the above steps, particularly the very first one. They will use filler over rusted sections rather than replacing metal. I had one of my cars done years ago and within a year the rust came back in exactly the same places. I took the car back and made them do those spots over. I was impressed when it only took them a day to do. Years later I replaced those panels completely and decided to grind down to see their work. Guess what? No new metal. They had just dented the rust and troweled on the filler. I have done all the prep on several of my own cars over the years since and never had rust return in the spots I worked on. It isn't difficult to do right, just very time-consuming and expensive. The last car I did completely cost me over $1000.00 just for the products I applied.
I agree fully with the people who suggest looking at the previous work of a shop. Talk to people who have had a variety of work done, not just insurance work. It's a rare shop that does rust work at all these days, as it just doesn't pay the bills. Friends of mine who specialized in restoration work just went out of business because nobody was willing to pay what proper rust prep and paint prep costs in time to do. $4000.00 if no rust work is involved isn't high at all for a good job. Best of luck.