Look for RUST.
If you plan (or hope) to keep the engine make sure it turns freely.
There are plenty of engine swaps available for this car, it's a 78 so not too many people will cry if it isn't kept "original".
(there will be some, let me warn you)
The car will go like a bat outta he** if you put in a Datsun 1500 with a high perf head and twin side draft carbs.
Mine has the Datsun motor, but the stock downdraft carb and I got a ticket a year ago for going 87 on the freeway. (I've since been a very good boy - mostly)
If you couple that with the Datsun 5 speed transmission you wont have to spend all the money on a conversion kit - money that can be spent on the motor.
Anyway, whatever you plan to do with the drive train - look for rust.
The rear spring boxes where the leaf springs connect to the body - those can be a killer if they are rusty.
There's no frame, so structural soundness is job 1. If it's rust free and they don't want more than about $750 I'd go for it. It would be worth twice that much with a running engine. A bit more if it's totally rust and dent free.
Be sure to check for bondo covering rusted out areas.
Be sure to look closely at the floors in front of and under the seats.
Here's how my 78 looks after a few modifications...