Look for a Parker 45 at about $25, give or take maybe $5. It's really the oldest of modern pens, but many EBay sellers seem to think anything made before 2000 is vintage. Some expect to sell a P-45 for as much as $100. Ignore those.
Parker produced the 45 from 1960 until about 2006 or '07, and it might have been the all-time biggest selling fountain pen.
The Parker 45 was the first successful cartridge / converter pen, and the first pen in which a user could unscrew all the major parts. A user no longer sent the pen to Parker service to fiddle with the filling system. Any Parker converter should work, and any cartridge. The P-45 nib unscrews, like the older Esterbrook.
A 45 might need a flush, but it should work without any trouble.
Here is Tony Fischier's description: https://parkerpens.net/parker45.html