Hi, Pin down a builder? I would suggest you take the plans you have to a builder and get some feedback before you go much further. Builders are often your best ally when it comes to practical ways to achieve the design intent.... many is the time we have been asked to price a set of plans where the client has invested an inordinate amount of time and money on detail only to receive a rude shock when the reality hits the budget for all the detail desired.... cheers

I would attach it to the vanity, it is awkward to try and reach behind you if you attach it to the wall or you could try a freestanding one that you could put anywhere though they might take up a bit of room when space is a bit tight

En-suite layout is fine, Looks like you have a double hung window with a slightly lower sill to the vanity height. . I’d put a stool in front next to shower screen. Update vanity and top and splash back, maybe lowering mirror. Could put a niche in the side wall if want extra storage. Could change mirror to a cabinet, even if it protrudes, mine does, A smaller basin would increase spaciousness and bench space. Can also increase depth of vanity, to 550. Replace floor tiles if you want but I’d leave shower if you want to keep costs down. otherwise retile to full wall height will improve look. Could add a pendant light over left side of vanity

I’m set up in our sunny spare bedroom. Happy with my spot as I have views to the street and a desk to work from.