Generally speaking, I suggest you don’t.
I know that everyone wants a bargain and equipment in poor physical condition is often bargain-priced. Sadly, there is no escaping the fact that equipment in poor condition tends to be less reliable and less repairable than similar gear in good condition. It also doesn’t hold value as well and is less collectible down the track.
If you are thinking you’ve snagged a bargain because you’ve found the make and model you’ve been looking for in rough condition, think again. Often, poor condition outside means poor condition inside, and poor maintenance in general. Corrosion is a good example. Once corrosion goes through equipment it can cause endless problems. Likewise, poorly cared-for gear is usually poorly serviced and maintained, even poorly repaired.
Despite the best efforts of skilled repairers later, these issues are usually not reversible. You cannot make a silk purse out of a sow’s ear, as they say, something to really keep in mind.