I need to replace my home air conditioning system. Nice down stairs but upstairs terrible. It's the original system, 19 years old.

My house is 2500 sq feet, 2 story.

What's the best system I should buy?

How many seer?

I don't want to overbuy but upstairs is unbearable!!

Not near enough information,, but you need a professional to come out and look at you situation,, they all are different,, he could do the measuring and look over you insulation/windows and see what you need,, also get more than one estimate, something to compare