It'll take a long time to pay off the cost difference between a mid efficiency and a high efficiency furnace. Generally, it's only worth going to a high efficiency if you are more concerned abut the environment, or you have other reasons in play.
Since it's a retrofit, you'll already have a chimeny, but if you were installing in a place that didn't have a chiney to use, you'd have to go with a high efficiency, as they don't require a chimney.
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