You're right to put the mouthpiece above the horn in your priorities.  For the classic sound you'll want a medium to large chamber with a low baffle.  The Meyer type pieces are very popular on alto.  At your level a tip opening in the range of .080" to .090" should work well.  You'll have to try different reed strengths to find out what works best for you.  I have no idea what you mean by different keys - it's the same for all altos, all tenors, etc.