Internal bathroom? Tips, pitfalls, costs?

[CarefulOwner]

Hi
We are going to be converting our current utility to a downstairs bathroom.
Some draining prep has been done, but we need to buy shower, loo, basin, and get a plumber to do pipework, build a stud wall, sort extractor? What else?
Can anyone give ANY kind of ball park figure for cost? We are going for homebase-type fittings etc, not villeroy and boch.
All help happily received