There is often some play in the shaft to Cutlass fit... so be careful he isn't "selling" something you don't need.
It sounds like the Cutlass is inside a strut, these can be pretty easy sometimes.... But if your rudder
is in the way of removing the prop shaft, maybe not.. it can be done without removing with the right tools and expertise. You might ask him his process..... and also ask for the total to remove the shaft and the cost of a new one just in case.... once he's got a "hook in you" it's too late...
If the boat sits quite a bit, when the shaft is out look it over good.... you may find it is pitted where the stuffing box is... so then you need a new shaft also.