I went to look at and test drive a '99 GC Laredo today (the one mentioned in a previous thread of mine).
The vehicle was down the street from the owner's office and he was busy, so he just gave me the keys and told me to take it for a spin.

But as I was walking out, he mentioned that it might not start because it had been sitting all weekend.
He was right, when I turned the key, all I got was a clicking noise.

I popped the hood, took a look, and noticed both battery terminals were corroded pretty badly.
When I went back, he assured me that the battery could just be cleaned up, and everything would be alright. He said the problem had occured before.
I know the battery had a connection and some power because I could run the air conditioner blower, the power windows, and the keyless entry system, but when I turn the ignition key, all I get is a bunch of clicking.
Does this problem seem as simple to fix as just a new battery, or is there something I'm missing? Something he's not telling me.
He has owned the Jeep since new and he said there were never any mechanical problems besides this corroded battery. I appreciate any thoughts you guys might have!
P.S. Will the battery voltage gauge on the dash read correctly with just the key turned on, or does the engine have to be running for an accurate reading?