For your radar needs (used), a JRC 1000/1500 will more than meet your stated needs, as would Raymarine.
JRC Radar 1000 LCD Complete Set Up Excellent Condition | eBay
JRC 1500 LCD Radar Display 18" Radome Wiring Free Shipping with Bin | eBay
Low Hours Raytheon Raymarine SL70 2KW Marine Radar System Complete w Dome Cable | eBay
You hit the nail on the head- getting practical training on radar usage is more important than the hardware. The latest gear uses the same operational principles as the older gear.
I'd advise looking at newer gear that is modular- that is, you can start with a display and radar, and add sounder, GPS, etc as time and funds dictate.
Maybe something like this:
Raymarine A67 5 7" Multifunction Radar Pack | eBay
Small, powerful and meets all needs of GPS, chartplotter, radar capability, built in sounder, and expandability.