I would like the flexibility to also add preamp outputs and additional front surround speakers and sub for a complete 7.2 setup in the future.
OK, no worries... save the sub conversation til you're ready.

I will say this: I'm only scratching the surface right now when it comes to integrating my subs, but truth be told, you really shouldn't even notice them in daily usage. Properly set up, it will give your speakers... gravitas. And depending on the extension the sub is capable of, the potential for some good infrasonic thump.
The latter... in an apartment with close neighbors, sharing walls and floors/ceilings... could be bad.

Beyond that, low frequency sound isn't directional like a tweeter can be, so down firing really seems to be a matter of preference more than anything else. Some people (here, even) have said they like a front firing sub just so they can watch the woofer. OTOH, I have also heard of putting a down firing sub on a platform so that the first surface it hits isn't padded carpet. I am employing that technique myself, by putting my X-13s on a platform, supported by spikes, and using isolation feet on the sub itself. I don't have close neighbors, but have a very spongy vibration-prone floor. The spikes are said to reduce surface area contact with the floor, while allowing better stability on med-pile carpet; the platform helps absorb some of the vibration (along with the feet) while eliminating carpet as the first point of contact. The best description of isolation feet I know is on SVS website for their
product.
Maybe I'm crazy... but so far its working
for me. My old RatShack LX5s with Linaeum tweeter and a 4.5" woofer sound like the double bass is in the room with me. Those speakers alone can't do that.
NOW>>>
back to the point of post #1...
Do you have the ability to listen to either of the speaker brands you are interested in? Have you considered ELAC, Fluance Signature?... so many other good options, too!
For the receiver, I would also recommend looking at the Denon models comparable to the Marantz... either the 3400 or 4400, I think. Little less gadgetry you might not need with sightly more robust amplification.
Lastly, consider upgrading in parts... maybe your front 3 speakers and the receiver now... then another pair or two in 5-6 months... That might allow you to buy a slightly better AVR now to anchor your system for the next 10 yrs.
Happy hunting!