both 5500 generators should be same...5.5HGJAB Onan. Hours run and how they were maintained more important than actual age. If they have less than 2000 hours and run well, then either one probably good deal for $.
I would want to see/hear them run and verify voltage output prior to purchasing. Is your Guardian a QP55G ? if so the Onan will fit in same space. I have done this swap numerous times. Takes a little skill to adapt the remote control wiring harness to new set, plus wiring and fuel lines on opposite ends of Generac and Onan
