Amana...I had two companies come out and both said 13 was the way to go....hmmmm. They both said it is probably a 2 ton unit, numbers on side are not readable anymore
As far as how noticeable its going to be on your electric bill, probably not much. 2 ton units really don't draw much current, so maybe 20 bucks a month tops during the summer (depending on your usage rate) assuming you've got an average amount of insulation. Amana isn't the most reliable brand out there but its generally a generic unit so parts are cheap. Don't get me wrong, it's not like the thing is going to break down all the time but its not gonna be a 20 year investment.
How old is your house? Plaster walls or drywall?