That will be an extraordinarily nice setup. I couldn't justify a 750 and 650 so I just got the 650 and left a legacy navcom (KX-155) in the panel, ultimately to be replaced with a GNC-255.
Generally I suggest the 345 with a 650, vs. a 345R with 750. The screen real estate on the 650 is small enough as it is. That said I still leave my "remote" transponder field up on it anyway, so I'm not practicing what I preach. Maybe one of these days I'll get around to reconfiguring the unit.
With a FlightStream 510 and an iPad you're going to find you won't need the actual screen of the unit that much. I control my 650 via Garmin Pilot and Foreflight. That includes flight plan transfer and route editing while I'm flying. If I get a re-route I just type the string into ForeFlight or GP and "send to panel." This then syncs everything else in the plane talking to the 650, including the other iPad. This is another reason why I went with the 650.
If you're comfortable with the ability to control the transponder tied to the health of the 750, there's no downside to the remote. Not a huge deal, and yes it is subjective, but I sort of like having the separate interface for the transponder. It also comes down to how much panel space you have. I could certainly see all sorts of different variants to this. And would I remote an audio panel to the 750 to save panel space? Absolutely. But these were installation extras that I couldn't justify when I did my big upgrade in '16. I just did the 650, GTX345 and a G5 ADI. This winter I'm planning to drop a G5 HSI in as well.
As nice as a 750 + 650 setup is do you really need a second GPS Navigator? I briefly considered dual 650s. It's a great setup if money is no object, but it's a hell of a lot of dough to plow into the panel to back up a GTN. If money isn't a determining factor, then by all means, it would be very nice indeed.
Congrats to you for going with Garmin. Although I am occasionally accused of being a "fanboy," what I'm truly a fan of is making smart business decisions when it comes to aviation tech purchases. If you're going to put that much money into a panel, make it a good investment and buy the market leading products. When people decide to go boutiquey and put off-brand stuff in that's not designed to talk to each other, it often contributes to a less than optimal result both in terms of how well the panel performs, how future-proof it is, and the residual value of an aircraft. 9 times out of 10 a top end Garmin panel is going to lead to a faster sale than the other brands. I also learned my lesson the hard way when I first updated my panel in '01, and decided to go with an all-King panel instead of Garmin. That turned out to be a very bad decision.
Although I also have and use ForeFlight, I do find myself using GP quite a bit in my own airplane - because it looks and feels exactly like the GTN. You might find the same to be true for you. Your iPad will feel, look, and act like an extension of the panel with the FS510. ForeFlight works great too, don't get me wrong, but it's not visually similar.
The Garmin GTN simulator is pretty good for learning the box. An actual course or two might also help. I found the 650 to be extraordinarily intuitive, but I still learned a lot of new stuff when I went to OSH in '17 and sat in on a couple of free Garmin GTN seminars.
Good luck,