Hey Eric,
As much as an 80cc would be a good way for a newbie to learn, you've already got karting experience in something faster. Although I haven't personally driven an 80, I do know they're significantly down on power and torque to a good 125.
The "great" deal is something to be careful of. If you have first hand knowledge of the kart and it's local, you might be fine, but otherwise, I wouldn't do it. If the former is true, you will have a great start to simply picking up a 125 motor for $1-$1.5k and you'll be set to go.
So you know, my wife started karting in a strong 125 shifter and she's doing fine. She did get a little indoor track experience first, but not much. You can always drive the 125 slower, rather than be restricted by the motor.
To give you a comparison, around here there are a couple of nice karts, already setup for F125, for $3000-$3500, total package. So, consider that when you're looking at the 80cc.
What year chassis is the 80cc? What is "tons of spares"? How's the kart been kept up?
To me, the bottom line is that you don't "need" to start with an 80cc, so unless the chassis is beautiful and you could simply throw the motor out and buy a 125, I'm not sure I'd go that route.
Brian
**3Generacing**
Craig: STX MINI Cooper S
Jane: A Stock Boxster S
Brian: F125 CTS/Champion Honda
Lisa: F125 CTS/Swedetech Honda
Julian: Intrepid Cadet/KT100
Carson: Top Kart Cadet/K80