Sounds like a nice idea to me- and you’re doing it right by getting a bike shop to do it rather than getting a mate to strip it and powder coat it. If you’re doing it as a surprise Argos will do a job he’ll be 100% happy with and get the right decals. It will look as if it came out of a factory, whereas a powder coat job and buying decals online will always look a little bit “fake”. Obviously you’ll know what colours he’ll like and so I doubt it will disappoint!
Bike shops are better with the phone, but the best thing to do is go in. You can leave a bike with a larger shop for a while (heck, I used to work in a tiny shop and people left their bikes with us for entire summers…) so if you do take it with you they may be able to keep it so you don’t have to do another trip. Taking the bike with you will show them you’re serious about spending the money- a lot of folk come in saying “oh, I might get it resprayed” and you don’t see them again- but don’t be too disappointed if they don’t have room to keep the bike- bikes are bulky thing and the shops often don’t have much space for those in for repair.
As for being a woman, it’s usually much nicer for a shop to work with women as they listen to what advice you give them and don’t bugger you about so much, so that’s not the issue.
The bits you’ve chosen should appeal to a Karate Monkey owner nicely, though I would say maybe go up to XT brakes- they have shiny bits on (as you’ll know, mountain bikers love shiny bits) and are a bit posher. No discernible performance benefit but more bling will mean a happier boy.
Let us know his reaction!