Now a days with DCC being so prevalent any good hobby shop worth their salt that sells trains either does repairs and dcc installations in house or has someone who does it for them. I've done quite a few for friends and club members and like I've told them and it's been mentioned by another poster in "some" cases it's not as difficult as you think. If yo go to the TCS link I've provided you will see picture how to section on various scales and manufactures. Like I have told the guys at the club who I've done them for if I can do it pretty much any one can. There are some locomotives that may require special tools or equipment etc.but thats usually for converting the old analog models to dcc. Pretty much any dcc loc can be easily made in to a dcc/sound unit.
Where do you live? Shipping costs something each way, and there is always the chance of loss or damage. If you can take it to a shop in your area and put it in someone's hands, and then pick it up when it's done, you may save money and you won't risk losing the engine in the mail.
I had sound added to an engine at Maine Trains in Chelmsford, MA. www.mainetrains.com. They did a great job for a great price. Normally, I do these things myself, but there was no room in this engine. I still don't know how they did it.
It takes an iron man to play with a toy iron horse.
Here's four:
Litchfield Station
Empire Northern Models
Ulrich Models
Tony's Train Exchange
All four do terrific work.
Tom
https://tstage9.wixsite.com/nyc-modeling
Time...It marches on...without ever turning around to see if anyone is even keeping in step.
You probably should list the loco including who made it, the approximate age, the model number and the decoder you're looking at putting in it.
Springfield PA