Well, if you MUST have a 76, your choices are limited...

G70, E60 (but a little limited, especially in the guitars - no insert effects), PAX or Ketron (and I'm sure abacus will point you to several others that cost $7k+, but you don't sound like cost is of no concern!!)...

Ketron does have the mp3 player, but some sounds are a little long in the tooth. Korg's new PA2Xpro sounds like a good one, good Guitar Mode (Korg's can be used in Style Mode, Roland is live play only) but limited fills (3 compared to Y's 6 and R's 7). G70 is heavy, in many senses of the word! Brilliant piano, B3 and drums, but no mp3 player.

For what you sound like you need, I'd give the PA2Xpro and the G70 most of my attention.

And forget the harmonizer. No matter HOW good, they ALL make you sound like cheese! I haven't heard a demo on ANY keyboard yet that I thought the vocals were realistic with the harmonizer. A pretty sick gimmick, IMO (but I'm not trying to cheese out like some here). Once again, try before you buy. This 'feature', once you realize it's limitations and uninspiring sound, may not turn out to be so important as you think it is.

For me, my priorities are inspiring drums first, inspiring piano second, and inspiring B3 third. This makes the G70, despite it's weight and no mp3 player the perfect choice FOR ME... It's not perfect for many, but my priorities are mostly sonic and tactile, rather than convenience, so it works great as an arranger AND a 'live band' main keyboard.

BTW, you REALLY want to tie yourself down to audio files that will probably stay with you LONG after you've moved on to better keyboards? Most don't bother with completely re-doing ALL their mp3's EVERY time they move to another arranger, so 75% of what your audience hears is the old arranger you got rid of. (But if you DO go the MP3 route, why not hire a REAL guitarist to play on the recordings, and make the arranger's guitar sounds moot?)

Bottom line.... ALL of your choices are FAR too expensive to purchase without a good trying out. Don't make the mistake many make and end up with buyer's remorse (and a hefty re-stocking fee!). PLAY it, and LOVE it before you buy.