the MR, so far, has been pretty tuner unfriendly. The trans has it's own separate computer that has to be reprogrammed as you mod, which no one that I've heard of has had any success at cracking the code on. Adding to this, its widely been speculated that the MR trans will not have a high threshold for holding hp. At this point in time, I'd only recommend the MR for light modding, and if you're a hp junkie (or your first turbo car), I'd get the non mr for simplicity's sake. Now's a great time to buy because no one is buying cars right now; dealers are pretty desperate and willing to come down quite a bit.
For your size, you'd be pretty snug in a Evo 8/9 but the 10 has a little more room.

Now let me get my plug in If you do decide to keep the Z and are interested in FI options, we have done quite a few at S&R, with built and stock motors alike. We just finished a 350z with a Greddy TT kit and built motor that turned out awesome

I think you'll be happy no matter what choice you make, all are great cars