I had the chassis from my '57 Oval blasted, and the firm concerned damaged it, not by the blasting process, by by carelss handling.
I had both the chassis and the car body blasted again, but we used recycled glass as a medium. This is much less harmful than sand and/or grit blasting in as much as there isn't the same heat build up. The glass can be reused, and it comes out finer each time. Bizarre as it seems, it works better once it's been used once, as it seems to cut through the old paint better.If there's any filler under the old paint, you'll need to concentrate on that area for longer.
If you are still going to do this yourself, my tips would be:
1)Start with a piece of scrap metal and work at it until you get the hang of the process.
2)Stand well back from the car, and work the blasting nozzle in a circular motion, so that you're not concentrating on the one small area. If you feel you're not removing enough paint/rust, take a step closer to the car, but move carefully.
3)Work round the edges of the panels first, only moving to the central, unstressed areas once you're sure of what you're doing.
4)Use a lower pressure until you see how it's going.
5)Stop regularly to check your work, and get some fresh air.
6)Take your time!
7)Clean all the media out with compressed air, and etch prime the job asap after it's done.
If you're in any doubt, find someone reputable, but ask lots of questions. Have they experience of blasting car bodies, what media do they use, can they use a media that's less harmful. What type of primer will they put on after blasting?
It might also be a good idea to actually be there while the work is done.
Good luck whatever you decide to do, and keep us posted!
Gavin.