I would go through the effort (or possibly extra cost) of finding exterior paint. You can often get it for free from a households toxic waste recycling facility if you go there, kindly ask the workers to save a few 5 gallon buckets of latex exterior paint for you. they generally don't accept latex paint (it's not toxic), but tell people to pour it out on kitty litter and throw it away.
is the (in my opinion good) possibility that you will have more labor costs in the form of having to (possibly strip &) repaint 2 years earlier than the saving of ten bucks on a gallon of paint? to me= no brainer
this is based upon my experience as a painter where a good quality paint over a crappy coat of paint = a crappy coat of paint that is instantly blistering.
yes you can slap on hundreds of gallons of paint but have you slapped on interior paint, outdoors, and then gone back and checked on it 10 years later? (I have not myself so I say,?, but it makes sense to me to spend the extra $, if necessary) I have come across many customers who were unhappy with the last person who painted their house...
, or try a post on craigslist, even calling contractors, etc I bet you can get old buckets of exterior for free