Firstly, There`s very few Gun Smiths I`de trust to do a Career Service. Because of the number of O rings involved and the complexity, the time it can take to find a leak, the price for a service will be at Least £100 and possibly more.
Value anything from £300 for a leaky rough gun to possibly £600 for an unmolested gun in perfect condition.
Because of the lack spares in the UK many sellers are dismantling and making more money selling parts. This can take many months though.
The inlet valve is an easy fix, information is out there. You`ll need to make a tool to get the end screws