Replacing furnace or whole system

Found out this wk that my furnace has a major problem (started tripping the flame rollout switch) and according to the ac repairman needs to be replaced due to a crack in the heat exchanger. He said he could not see a crack but determined there was one by process of elimination (checked the gas pressure at 3.0in h20 when running and 6.5in when stopped, checked the exhaust for blockage by removing the vent temporarily while running, and cleaned the metal gas channels between the incoming line and the heat exchanger). When running, the front of the furnace gets too hot to touch, so I also know it’s not the sensor itself.

Anyway they told me that after first of the yr any new systems installed will have to meet higher efficiency ratings (16 seer), and if I replace only the furnace now, and if I later have to replace the outside unit, it’s not clear how compatible they will be. So I’m left choosing whether to replace the furnace (appx 3500) or the whole system (appx 10500). The old system has a mfr date of 2010. Also if it’s relevant it looks like some prev work was done that is kind of dubious (one of the two rollout switches was not present and I guess is jumpered across), and I replaced the contactor on the outside compressor earlier this summer bc it went bad.

I’m looking for advice on what to do. Furnace or whole system? Get a second opinion?