Unfortunately, running well means nothing with El Primeros, so you will want to factor in the cost of a service, if you plan to use it. That will likely set you back $500-700 with parts extra.
I sent one in for service last year that kept perfect time and had very good amplitude on a time grapher. It had been services 6 years earlier by Zenith and I had owned it for a year. According to my experienced watchmaker, there were a number of lubrication points that were bone dry and incurring wear. One of these was a post on the main plate that cannot be repaired without changing the entire main plate. I was lucky in that it was still usable.