what type of door do you plan on using? UPVC,composite etc.. or just a regular wooden fire door?
taking a upvc/composite framed door out whole and spinning it round is pretty straightforward for your application as you don't have to worry about how the drainage is setup as its inside. a basic composite would be around £500 without all the frills if you know where to shop, just make sure its double rebated. although I think it would need to be fire rated.