BiggerPockets

The concern I would have is on the life expectancy of furnaces. Average life expectancy is given at around 20 years (see link below), and you know your used ones are already more than halfway there. If you have better than average items, could be a money saver; if you have below average items, you could be spending more on the replacement labor than you wish.

If the landlord pays the heat, many newer furnaces are high efficiency so that you could gain energy savings with new models.

http://www.pillartopost.com/home-repair-cost-upgrade-guide.aspx

I got an early model High Effic. Furnace once for $100. Luckily, it fit the plenum and intake from the old one without modification, and the only new part was the PVC exhaust as I was replacing an 80%er. Got a guy to install it for $150. Then I had to get another guy in when the fan motor appeared to burn out (froze up and wouldn't turn). He fixed that with some kind of bearing lube, but found several other items and had to get parts, had to redo the exhaust from the first guy and in all, I might have saved $300-500 over the cost of new High Effic. Not worth the trips and what felt like Chinese water torture trying to get all the bugs out. It's worked flawlessly since, but still wish I'd have just gone new.

i like the older furnaces better.. they put out hotter heat and are super simple to fix. I really don't see much improvement from swapping out a old one to a new.. windows, air leaks, insulation, ec. would be better to seal up.

i can get new furnaces from 400-500 and i install them.. The brand new furnaces are MUCH simplier then the70's- 80's era furnaces.

I would stay away from anything but brand new or just keep your old one... which is what i would do.