If it has rust on the underside you can't have it undersealed anymore so have the garage where you have it serviced spray the underside with a thin layer of waste-oil when you have it serviced (6 monthly), as I did with my 16 year-old Ford Cortina, it will help to protect it against further rust - my mechanic who also drives one told me this tip. Or do it yourself with a paintbrush and leave it overnight to stand. And treat any other rust you find immediately.
It's rust that tends to kill them in the long run. The engines usually go on forever if properly maintained. The reason old fords rust so easily is because unlike modern cars they were not sealed on the inside surfaces of the car and so tend to rust from the insides out, if you see what I mean. Try and keep the interior free from condensation and damp as much as possible. Keep it in a garage if possible.
The Cortina was my Dads baby as well. He had it from new then when he died my mum learnt to drive it (they put up warnings when she went out ). When she got a new automatic (thank God!) she gave it to my sister and then it came to me.
I passed it on to a friend who loves Cortinas when I bought my new car when it was 18 yo ~ it's 20 this year and still going strong. The bodywork is in remarkably good condition considering it has spent the last 6 years outside. Mileage was less than 50,000. The mileage on my present car is only 14,500 though which for nearly 5 yo is very low.
This truck will remain in our family. My mom would have a FIT otherwise. My dad bought it new, when he was 17 years old. It is the truck my mom learned to drive in. Now that my dad has died my mom is getting it all fixed up again (he was mid fixe up when he got sick) and she hopes to beable to drive it next year.