The stock 240Z ride height is aorund 8" and a really low, good handling 240Z has about a 5" ride height (measured at the rockers). A 3" drop would be plenty for your goals.
Print out the Strut Sectioning FAQ.
Good choice with 225/50-15 tires. Probably the best tire and wheel size combination for a 240Z.
That will get you 1" of the 3" total drop you shoudl shoot for. The remaining 2" comes from the sectioning and ride height adjustment on the threaded lower spring perch.
Print out the Strut FAQ.
Money is always an issue, even if you have a lot of it. I charge $500 to disassemble, clean, media blast, cut, weld, weld on the lower threaded collar ring, and paint a set of 240Z struts. You can do most of that yourself and save about $400 of that if you have a welder just do the welding for you.