I've had 3.5R for 12 years or so. They are hard to integrate seamlessly with sub woofer due to the speaker being dipole, it's roll off and room mods.
1 sub or 2 depends on what frequency you will cross. I am not intimately familiar with MMG but if you cross anywhere over 130hz, 1 sub will create a problem with imaging. You will simply hear sub and the speakers not a coherent seamless system.
If you are listening to 2 channel stereo, I wouldn't bother with anything less then 1.6 or whatever the current model is. MG 20 is even better but pricey.
In terms of power, more the better. They like juice.
What kind of sub? I'd go with H or U frame dipole sub but I don't know if any are available on the commercial market. I would guess that acoustic suspension would work out easier with Maggies.
Parts Express sells a nice Dayton Reference kit. My friend just build one and it's great for money.
I think Velodines are pretty good too.