I think my first question would be why is it cracking, and where is it cracking. Top Section, center? Steel garage doors can be painted, after a good cleaning, with a GOOD quality latex paint. If your opener is working well.. then yes you can replace just the door and keep the existing op. Might be a little difficult to diagnose whether an op is "about to go". For me, it would depend on how it has been running, the name brand and the age of the opener. The co. I work for sells a few different brands of openers(including our own brand) and I service all of them plus one's we don't sell. I get the fewest service calls on Liftmaster Ops. NO, I do not work for Liftmaster, but I do have one in my garage :)

I agree with the rest, keep the machine, get it tuned, and get a new foot pedal--popping and you could have a short in the cord because the cord is that old, could be hazardous so I wouldn't run it until you get it fixed. That was probably what was wrong with my 1963 Lady Kenmore purchased new, and you just made me realize that may have been the problem after 30 years of pedal to the metal--it and the blonde formica cabinet went to the dumpster during a move. Also, if you are running the pedal on carpet, put it on a hard surface such as a cutting board (I have plenty of them, so it works for me). I fried a $65 Singer foot control because I had it on carpet (it would start up, run and speed would slowly die). I now always have a board under the foot controller...I have mostly carpet. You are lucky you have the instruction manual...some aren't that lucky. Sears still parts and the extra feet for your machine, if you don't have those, for around $29 excluding the ruffler...the original box had the ruffler included, or you can buy any after-market low bar foot that screws on or has an adapter. You can go into Searsparts.com and look your machine up to see what parts and feet are available. Your whole model number should be on the electrical plate on the back and in your manual.

If you live in a cold climate, insulated doors make a big difference in winter and summer. I replaced an old sheet metal door with a double steel wall insulated one and it's the best money I ever spent. If you do replace all 3 please try to make sure they don't get scrapped. Donate them to the Habitat for Humanity Restore or a similar place.

If the job is planned out in advance, it should take about one day to remove the door and hardware, frame and install the new door and trim. Painting would take an appropriate amount of time to allow drying between coats. If drywalling is needed on the inside, that would also take some additional time. If you want custom windows or other accent to make up the difference in width between the garage door and the french doors, that would also take more time.