I did the 20 inch, RonJon Legacy Type II Custom Graphite face with machine lip, Continental DWS 06 P255/45ZR-20 along with wheel locks (ronjon locks + gorilla locks). I also did Racing brake slotted/vented rotors, steel braided brake lines, and Carbotech 1521 pads for the big girl. Converted the old 19" advance rim into a full size spare. Zero issues with the combo after almost 5 years.
I did the 18" Legacy 7 Liquid Graphite for my RDX. You might have to call to see if they can make the rim in the MDX bolt pattern if you like the Legacy 7.
I second Discount Tire Direct. Call them and speak with Don. He is a technically savvy guy who can do all sorts of magic with pricing and shipping.
Cross shop the rims you like, and you will be set. Carid, AutoAnything, etc carry some decent wheels and do have properly selected (load rated) wheels for the MDX.
As always, pick a reputable aftermarket/OEM manufacturer like TSW, Enkei, Motegi, BBS, etc. Buying cheaply made wheels that may not be cast properly, will rear its ugly head down the road (no pun intended). Just dont be satisfied that just because the wheel fits and bolts up to your MDX, it is made for it. The load rating is very important so buying from informed stores is always helpful. A lot of times, your local tire store will be able to provide you sound advice and also price match anything from online.
I use this partly for wheel dimensions, but I also largely use this to see how much my speed will be affected by the upsizing. Depending on the combination of wheel and tire size you upsize to, you can then see how far off your speeds would be from OEM. Personally, I try to keep it as close to the same speed as the OEM.
Also consider that as you go to larger wheels, that's more rolling mass to get up to speed, as well as slow down.