I think yes, it is reasonable. A TEFL course will help you get into the way of teaching. Of course nothing beats experience but it will help a lot and you won't walk into the first class completely lost.

There are some reputable online courses; and yes there are some rubbish ones out there so it's just a matter of looking around till you find one which you feel comfortable with. Of course you should send off any number of questions to the company involved and see how, when and if they respond.