Rust as any chemical reaction will have higher rates at elevated temperatures. It is often falsely believed that your car rusts during the winter time when there is salt on the roads. Actually, your car rusts after the winter, when the car's frame and undercarriage are covered with salt but when the temperatures are elevated.
So, to prevent your car from rusting again, I would advise you to do several things:
- Purchase your cars as new. Old cars have already rust and there's nothing you can do to prevent it from expanding.
- Get a rust protection job for your car. Immediately! As the first thing you do with your car! Expect to pay around 1000 USD, mostly for labor. A good job takes about three days. In areas where salt is often used on roads, rust protection shops are prevalent (at least in Finland I know there are many rust protection shops near the capital area).
- Take your car for a rust protection shop for inspection once per three years. Let them do whatever fixes they think are necessary.
- Wash your car very thoroughly immediately after winter, when salt is no longer used on the roads but when temperatures are not yet elevated like during the summer. This prevents the salt from causing the chemical reaction that turns your car to rust.
Now, you said you bought a 20 years old car. Car of that age is expected to have lots of rust already. Did you inspect it thoroughly for rust with a flashlight under the car? If not, I would say the expected lifetime of your car is about 2 or 3 years based on the information that I have. A 20 years old car is going to have other problems than rust as well, e.g. you can expect pretty much everything made from rubber (oil seals in the engine and in the power steering rack, fuel lines) to fail.
If you really, really want to buy second hand cars, do buy from a place a bit further away where salt is not used on the roads and thus rust is not an issue. Even then, you should verify the car thoroughly for rust. Then let a reputable shop do the rust protection job. Note that for old cars, you may need to pay extra in addition to the usual price of about 1000 USD. This is because they may need to do something for the rust that is already present.
Is the meticulous care against rust worth it? I would say it is, especially if you prefer larger and more expensive cars. My car cost about 50 000 USD. I expect to get about 20 years of lifetime from it, thus costing 2500 USD per year. A 300 USD rust check once per 3 years adds 100 USD per year. A 1000 USD rust protection job adds to the price resulting in 51 000 USD, but if my car now lasts for 25 years, it's 2040 USD per year plus 100 USD per year or 2140 USD per year. Somewhat cheaper that way: you save 360 USD per year.
Now, consider a small car that costs 17 000 USD and requires a 700 USD rust job (cheaper due to needing less chemicals) and 300 USD rust check once per 3 years. It's 850 USD per year if the car lasts 20 years without rust protection, and 808 USD per year if the car lasts 25 years with rust protection and with regular inspections. Now you save only 42 USD per year. Is it worth it? Probably not, considering that rust is not the only enemy of old cars. Your car may fail before rust starts to be an issue.
These are prices converted from Finnish prices using the conversion 1 EUR = 1 USD. In areas where taxation is lower, prices may be lower as well.