I avoid the Beechcraft 1900 whenever possible. I know it's bad to make generalizations, but I don't trust the 3rd party airlines that operate/repair them for the major airlines. Also, if you do end up on this flight, take earplugs.
Anyway, the airline may be able to re-rout you to jet-only service for little or no extra cost if you explain (and can document) your medical condition--I doubt they want a plane taken out of service after you fly in it
And, I'm not kidding here, are adult diapers adequate for IBS sufferers? What steps can you take to temporarily minimize IBS? What if you take a medication that'll make you really sleepy so you can just sleep through the flight, or slow down your digestive tract?
And lastly: Expedia
does show you the equipment type before you buy the ticket. It's listed at "Step 1: Review the FLight Details."