What type of diagnostic tool or tools should i buy for our 5 cars?

we own an 85 oldsmobile toronado, 89 Chevi. Camero Coupe, 91 Grand Pri, 83 Ford F150, 98 Chevi Van,and a 98 Buick Regal. I'd like to buy a diagnostic tool to help my husband know when things are wrong with them. He fixes them himself most of the time but some times he has to take it to the shop. Can i buy one or two to fit most of these? and i'd like something that is very thorough with engine/motor problem detection. any ideas? I'm at a loss! I am hoping to get him a couple for Christmas.