I just sandblasted my two front quarters, both doors, both headlight buckets, and the cowl. I wouldn't worry about denting it unless you have a high PSI, if you hold it back far enough and don't stay in one spot you'll be fine. Just always keep it moving or stop. DO NOT use silica sand, when airborne it can cause problems if inhaled. I recommend using coal slag, Black Magnum is a good brand.
I go to this place called Consolidated Stripping and do it myself for $15 ($45 for them to do it for you) an hour. They have a few full size blast cabinets and a blasting room :
http://www.consolidatedstripping.com/
EDIT: I see your in Illinois, depending on where you're at, consolidated stripping is in Plainwell, MI which is a little north of Kalamazoo. It would be a drive, but might be a good option to consider.
I recommend looking for a stripping place in your area, often they rent out their cabinets when they aren't using them.
Well worth it for a few reasons:
1. No cleanup
2. High quality equipment + all the proper equipment
3. They have an air compressor larger than you can ever dream of
4. The air handling system to clear the cabinet is nice
5. Expert advice on what type of media to use
Took me and a friend both blasting from 10am-4pm to get the above done. Why? Well my car had 3 coats of paint, the original blue which came off easy, then yellow, then a white in which they layered it on very thick.
After all was said and done they charged me $100 out the door. No dents, just bare metal
Here is a pic of my door after blasting:
http://smg.photobucket.com/albums/v201/netrix2002/Datsun/?action=view¤t=PICT0013.jpg
Here is a pic of my fender after blasting:
http://smg.photobucket.com/albums/v201/netrix2002/Datsun/?action=view¤t=PICT0014.jpg
Unless you know you have a rusty underside or your going for a showroom restoration or the undercoating is falling off, I wouldn't bother blasting the undercoating. Just find the spots where its gone and spray over the old stuff. As long as its still there it will do its job, removing undercoating is very time consuming and messy. The above post by JIM73240Z is a testament as to how well undercoating sticks =)