I can't see any reason to go to ATP over a local school you're happy with. The advantage of ATP is it's quick and you fly twins more (though safety pilot time and sim time make up the majority of it). The twin time looks good to the airlines hiring low time pilots (which isn't happening much at the moment).
To get into 135 medivac stuff as PIC, you'll need 135 mins and probably 200 to 500 multi. Plus, you'll need an in at the company. Is the company part of a flight school/FBO? If so, train there. If not, it doesn't really matter where you train. If your happy with your school now, no reason to leave. Get your ratings up through CFI and instruct to build up to 135 mins, that's 1200 total. Then, you can go out and get a 135 freight job to build the multi time. Once you meet the mins the medivac place is looking for, you should have a good shot at it if you've made some friends there.