I guess the reason you are asking these questions is because you're not a mechanical engineering sort, and its nice to see someone asking instead of just winging it
I would say you are probably near or over 750KGS in which case brakes are a legal requirement.
Even if you are under 750KGS then you may need brakes if your tow vehicle isn't rated to tow 750kgs un-braked or over 1500KGS unladen
Also if the trailer MAM is over 750KGS then you need brakes even if the combination weighs less.
If brakes are fitted then they should legally be fully functional.
In short JUST FIX THE BRAKES
Deffo get it serviced, salt water and wheel bearings are a recipe for disaster!
If you don't know how to check the bearings get someone to show you how to do a quick bearing check.
As for the trailer MAM don't know, the chassis no may be stamped on it somewhere other than the plate though, and will be traceable