When you are working with vintage and newer components, an A/V analog-digital converter can be handy. I have a SONY DVMC-DA2 that converts between analog like composite & s-video and firewire. It’s a stand-alone piece, system independent, no software, etc. There are other models like it. Canopus ADVC, Dazzle Multimedia Hollywood DV-Bridge, Formac Studio converter & Power R Director’s Cut ADC for example.
You can also find various video converters that support HDMI for newer monitor options.
https://www.ifixit.com/ has many useful tear-down guides if you want to get your hands dirty with hacks & upgrades. There are guides for various iMac models.
You should see some of the frankenstein work people do trying to make a DSLR into a usable video production camera. Working with vintage gear can be just as much fun and often a more challenging learning experience if you get under the hood of how things work.
