Suitability as a highway GT car is mostly in the eye (ear & butt) of the beholder... what some folks are happy with would drive others nuts.
We had a PRHT NC Miata for several years. Wife's DD, and took it on a number of road trips and to trackdays. With aftermarket suspension and larger/stickier tires, it's doable, but noisy on the highway. Fantastic backroad fun, though not great on power. A bit on the cramped side for touring.
The NC replaced a C6, that was absolutely the worst POS car I ever owned. I think we got a lemon, but it was not an enjoyable ownership experience. There were a lot of design and component quality issues that were pretty irritating. As a GT car, it was pretty good, if a little on the loud side.
Prior to the C6, we test drove the Twins and 370Zs. Neither really did it for us, for a lot of the reasons mentioned already. The seats in the 370Z were VERY uncomfortable, but again that's an individual thing. The 370Z Roadster has enough trunk space for 3-4 computer keyboards, as long as you stack them carefully.
We also drove a Genesis Coupe... Hyundai is getting there, but it was a little heavier/slower/softer than we wanted. Kind of like a V6 Mustang.
And that brings me to Mustangs. Go drive a '15 up Mustang GT with the Performance Pack. Used examples should be dropping into your price range. We just traded our NC for a '17 Mustang GT, and I've been very impressed with the total package so far. The interior is still a little on the cheap side, but the driving experience is really good. Just make sure you drive a Performance Pack... it has stiffer suspension, Brembos, different vehicle stability tuning, and steeper axle ratio.
And a word on depreciated German finery... you might get lucky, you might not. There's some great values, but you could spend a lot of money keeping them on the road...