Is it worth it to repair or is it better to buy?
Here's a few things you could do-  


Get a new seat and seat post.  
Ride the bike and see if anything sounds or feels amiss. Places to look out for are the moving arts like headset(handle), the drive train and the brakes.   
The drive train seems to be in prime condition except for rust. Spray some WD-40 or equivalent rust remover on anything thats rusty and make sure you wipe off the extra cleanly. Then lube the whole drive train.  
Your brakes will need replacing for sure, best not to take risks with safety.  


You will have to decide the worthwhileness of the project for yourself. Money wise it won't cost much more than $50 (~20 for the seat, and the rest for brake pads and other implements like WD-40 if you don't have them). I would personally do it less to save money but more for the learning that would be gained from doing it. But the importance given to learning experiences differ from person to person and its up to you