The problem with what you are proposing is that you are expecting the job to go smoothly and be left with a running car after "x" hours or so work by the machanic.
In reality, the machanic has no way of knowing that the turbos you have are indeed a true "bolt-on" affair and that ALL of the required parts (including gaskets, crush washers etc) are available for him. There is also the possibility that exhaust studs may snap or he may find other issues that should logically be dealt with (eg water or oil connectors leaking) at the same time.
In an ideal world, it would all go smoothly as you hope. In reality there will probably be minor "issues" that crop up that are best (and can be quickly) dealt with in fully equipped workshop.