What is involved in replacing a missing speed selector on my two speed hammer drill?
I imagine is pretty simple and inexpensive


Spare parts for older machines are often either unavailable or surprisingly expensive (e.g. armature £37). 


  is this repair more complex than I envision?


You'd need to open up the case and see if there is any other internal damage.

You might need to fabricate or improvise a speed selector. For example you might use polymorph or adapt a metal part.

A new Bosch drill of that sort can be purchased for £35-£50 (PSB500RE PSB680RE are often available on special offer at 30% lower price). A cheap store-brand drill might be £20 or less.

Links and prices are illustrative examples, a web-search will find local equiavalents.