Go!
We had an E30 and E9x discussion, but the E46 M3 is a great car to compare. Lighter than the E9x, and more powerful than the E30M3 - the E46 has 333hp, and is said to be very tossable. Luxury ride with all the bells and whistles, then turn off the electronic nannies and hit the track!
Prices are around $17-22k for the coupe or vert - so cheaper than the BRZ, and available now (no waiting). Issues? Sure - maintaining a BMW out of warranty is no fun task. Buy a new BRZ, drive it 80k miles without any issues (hopefully!). Buy a used M3 with 80k miles that had no issues and see how your luck is as you approach 160k miles.
That said, a well kept E46 M3 may be a good "in between" vehicle. Buy one with 70k miles, drive it for a year or two, sell it with 85k miles for a couple thousand bucks less - then buy a BRZ when they are easy to get (dealers stocked).