Morning, hope the following information helps you regarding propellors
I've had a 58 foot boat for 10 years now. about 7 years hit some concrete under the waterline and bent all three baldes of the prop. Driving the boat home also ruined the stern gland but thats another story. i had a second hand prop fitted, the cost was about £100, but it wasn't very good. I eventually took the ruined prop to Crowthers in Oldham who refurbished, balnced and fettled it back to new for £50-£60. i then refitted this the next time the boat was out of the water. since then i have had the replacement prop balanced, re pitched to 20 x 15 (from 20 x 14) and keep it as a spare. so , they are very durable, can be repaired in a few days not expensive. New, i understand they around £250.
as regards what size, it depends on the boat length, size, clearnace under the counter and the engine and gearbox combination.
hope that helps.