Yeah, my guess is the battery had a decent charge when you bought it and now doesn't. A big issue with these cars, with age, is the big wire coming off the alternator goes through the firewall and then to the ammeter then the other side of the ammeter comes back out the firewall to the post on the solenoid tied into + side of battery. The ammeter is suspect and the connections at the firewall also can be bad.
Easiest thing to narrow it down is take your battery in to get tested. If it's good. Charge it up and see if the car will run
If it runs for a bit and then dies...likely bad alternator or wiring going back to battery from alternator.
Voltmeter- get one. It will be your friend for life. Can figure all this out pretty quick with one
