I certainly understand your frustration. But put emotions aside and concentrate what they can install and what their warranty is. Stay focused on what matters most... a dependable furnace that delivers adequate heat. Since you're not where the temps dip well below freezing, even an 80% efficient furnace would be fine. Remember, that last 10% to go from 80 to 90% efficient is derived from condensing the water vapor in the flue gasses. So you need a drain system and they can cause problems unless maintained. Again, the simplest furnace will be the most reliable.
Once you find the furnace you feel will be meet your needs, then you need to read the fine print in the installer's (not manufacturer's) warranty. Will they cover parts AND labor if there are problems. Usually a defective part is covered by the manufacturer but the labor to make the repair is on the homeowner. Again, read how well the installer covers your furnace. The better his commitment, the more likely he will do a better job installing your furnace.