I've talked to lots of mediablast places.....unless you have an idiot with the nozzle that has no clue on how to work with sheetmetal, you'll be fine. Places that do cars and have experienced people will do just fine..... don't take it to a place that focuses on big industrial jobs where the material is inches think etc etc. or they have some 18 year old kid doing it
The warpage thing is way overplayed....an experienced shop with the right material WILL not warp anything.
Soda doesnt get rust and there are plenty of issues with getting the metal ready for paint etc......