How expensive/extensive is this plumbing job?

You don't need to open your wall, you can just replace the shower body. To do it you would need to use offset connectors which you can get from the same manufacturer, an example of the connectors is here http://www.barwil.co.uk/Docs/pages2-3.pdf.
The spray and shower are piped from the exterior so you don't need to run any piping behind the wall, you just need to attach the to the wall using screws and anchors. Very simple.

You will have to open the wall, repair the wall, retile the wall and then the plumber will install. Your building might require you to replace all the lines back to the riser once you open the wall which would increase your cost.

This will cost you around $3,500-$5,000

grabs popcorn....

Primer, why can I not use the offset connectors as Arx describes?

I just called Barber Wilsons and they said they do not make offset connectors, and the shower body must sit flush to the wall and connectors would prevent this from happening.

I didn't mean adding additional connectors but replacing the existing ones with the ones that are curved the way the connectors from my link above are curved, this let's you play with the inlet spread. I would be surprised if BW doesn't have these.

I spoke to one plumber this morning who told me you have to open the wall. I will ask others on Monday as well as my plumbing suppliers