The solar panel technology has evolved in leaps and bounds in the past 20 years. Current panels are more efficient and resilient than old ones. I'd recommend saving for a new installation if you can.
In some places you can get a loan against your house equity for things like a solar panel installation if you install it on the roof. The thinking there bejng that the value of the house increases as a result. There may also be subsidies you can access.
That being said, you can test an used solar panel with a decent multimeter, by measuring the voltage (DC volts), short circuit current (DC ampers) and operating current if you can (this one is easier with a clamp meter, but you have to measure it on a live installation). As long as the values are reasonably close to the panel spec, you should be fine. There are tutorials out there on how to actually do it without blowing the fuse on your tools.
Dammit Jim!