Re: replacing the impeller.
if yer dropping the lower only, there ain't gonna be much other than the impeller to see anyways...... if yer dropping the lower only, have 2 peoples on hand to put it back, it's heavy and akward to handle. if ya take the drive off, ya will have more inspections that can be done at that point + it'll be easier to put the upper and lower together on the bench.
i agree with dons, take it apart and see what ya need for parts 1st. i used to buy up parts so i could be hip right away. but with my boats. one thing leads to another and i end up with parts that i've not used...... do get mercruiser parts if ya can.... they're what ya need...... get a service manual too. #14 i think....