Famous last words; but surely smashing something out ought not to go too wrong, wild flailing and catching other things aside: so my gut reaction is that even is a mess was made of putting in the new, one ought not be paying double to sort it out. Any additional labour taking out the messed up bits ought to be covered by savings having removed the shower yourselves. Maybe.
That said, in not familiar with the finishing touches, like the tiling and stuff, may pay you to get someone to do that bit. It's not too difficult if you take your time, but if you don't get it right you end up having to live with it.