Firstly, ignore any fly by night mobile outfits. As for the med17, most competent tuners have these cracked now, the maps arent any more difficult to identify and modify than most other ecu's.
Custom mapping is a process of testing, measuring, adjusting, testing, measuring adjusting etc.... So pick someone with the right tools, i.e a modern dyno and datalogging / tuning software etc.....
I spent 5 hours writing a map for a heavily modded vauxhall today, it had already been 'mapped' by someone that had very little experience or knowledge, or both as it was a complete mess. Im still not 100% finished with it, so dont expect a 'fast fit' type service.
Call your selection of tuners, ask the obvious questions, discuss your expectations and outline your requirements. The tuner that seems the most competent and has the right equipment might be worth a try, visit their premises and observe their practices if possible, if you are happy with what you see then give them the job.