You'll probably need 2-3000 hours at least with varying expreiences before even getting a look in for air ambulance flying. Get the FAA IR while you're over there, then do the conversion when you come back. You'll need to do the theory exams over here if they've run out but you'll then just do the type conversion onto the 355, and an abreviated course (dependant on ability and assessment) before taking the IRT.
You may be exceptionally lucky to get onshore IR work but not likely with low hours. (I only know of one person so far who's managed it, but he's a lucky guy
)
Not many people using the 300 over in UK. Try to get R22, R44, B206 maybe EC120 to help you on your way.