[tsubaki]

Sometimes it's more cost effective to pay someone to find the correct one.
When my neighbor bought his Nitro it came with a Rapture prop (pretty)(have forgotten the numbers on the props, so don't cretique this)with a 21" pitch. The boat would do 52mph but it took 12 minutes (not really) to carefully get on plane without ventilating. The "mechanics" arrived that it needed a 19" pitch and installed it. This time it would quickly plane and have no ventilation problems but only do 43mph.
Wanting the best of both worlds, I peed him off and told him to get a jack plate (so as to be able to dig the foot deeper when starting on plane) and put the haul *** prop back on it.
He's considering it.
There are a lot of variables in choosing a prop, you'll be more satisfied finding the right one yourself but it is sure to cost you before it's over with. Paying someone gives you the convience of complaining and getting what you want and some experiance, if you remember the numbers.
And the obvious, aluminum is fine, stainless is usually better, 4 blades usually give better performance.
Remember the escape clause "usually". ;D
Oh and the same sized prop in AL will not necessarily perform the same way in SS. LOTS OF VARIABLES
Sometimes a "cupped" prop is preferred for performance, yet sometimes the smooth tipped ones do better.