So your mechanic has done gobs of 5.3 swaps yet you are having to source your own engine? Does he not have a line on where to source some from?
At the end of the day, you will have upgraded your truck, but its not really an upgrade when you can just go buy that option from the factory. Just like dropping a V8 into a V6 camaro or trying to put a Cummins in a gasser Dodge.
You better well get a written contract and scope of work as to what your 10k is buying you with your mechanic. Have to talked to any references yet?
I agree with your approach to upgrade, but I don't think its just as easy as writing a check as your mechanic has led you to believe. If it were that simple, you would already see several vendors offering upgrade installations and at least some upgrade "parts kits" available. Maybe the demand is just too low, I don't know.
I've done an engine swap, I can tell you from experience, the engine itself will only be about half of your material cost. Its all the accessories and things required to get it hooked up correctly that add up quick. This is not even counting the labor costs.
Good luck with this.