I have a coleman evcon dgat070bde furnace and we want to add central air. (2 summers ago husband bought unit off craig's list used and the person selling it came out and installed an a-coil and a drain something or other, I think the unit it only a 1 ton anyway it cooled somewhat the 1st summer but nothing last summer) My husband has found a trane x80 (10 years old) furnace and a/c for 500.00 3 ton. Should we just buy a new a/c system. If so what size 16 x80 mobile home 1280 sq ft.

Never seen a 1 ton residential A/C before. You will need at a least 3 ton unit, in most localities of the USA. Often you will also need an upgraded blower package, for many mobile home furnaces if it was not ordered originally for an A/C option. BTW you can't install a residential furnace in a mobile home.