Nothing usually beats actually matching the burner to the actual load. (And yours could easily be under 50KBTU/hour on design-day, if not right now, after you've spent short money on fixing the most eggregious air leaks and attic insulation.)
Low mass systems like hot air furnaces don't suffer quite so badly as higher mass systems when oversized, but your unbeatable deal may not look so great when you look at it's de-rated efficiency due to oversizing, in a 5 year cost analysis.
OTOH, if it's a sealed combustion model (taking outdoor air into the burner, not room air) it may in fact do slightly better if only 2x oversized:
(See figure 6, the Induced Draft BONN 85 curve.)
If it's not sealed combustion, it'll be operating somewhere below it's rated thermal efficiency whenever it's cycling on/off. It's derating will be something similar to those other curves (but at least the 1.0 factor line is ~92-95%, not 80%, so it can be 3x oversized and do about as well or better than a well-matched 78% AFUE model.)
Since you won't be modulating, stepping up/down much, it'll likely see more ignition cycles than if perfectly matched & running fairly continuously in mid-winter, which will wear it out a bit quicker. If there's a way to add some hysteresis into the controls it'll last longer and run more efficiently delivering the partial-load heat with longer but fewer cycles.
I'm assuming you've already figured out where/how you'll be venting the exhaust and disposal of the condensate? If not, take a big step back. You can't just hook this thing up to a masonry chimney where the old one was and pass inspection. (The flue condensate of high-efficiency burners would destroy the chimney.) If the chimney has the right liner and isn't too long you might be able to use it though. See if you can't find the installation manual for the unit- it'll probably have most of the necessary info for getting it right, but it's not usually a DIY project for the uninitiated.
The price of the unit itself is only one factor in the installation cost...