Kim,
You sound like a good candidate for Contax C/Y, and for reasons of weight and features I you should consider three bodies: RTSIII, RX, and Aria. Your lens choice is in the CZ sweet spot.
Some responses to your initial posting:
- Camera repair and parts are legally required to remain available for a decade after product discontinuation. RTSIII is a current product, and so is the Aria, while RX(I) may have been just discontinued. Expect plety of life from C/Y, whatever Kyocera does.
- DOF locks exposure setting.
- I hate the traditional Contax display of shutter speed, with overlay of speed as set and speed as metered: a blinking, confusing mess. My solution is Av and AEL. Only when I can shoot (and think) slowly do I use M. In contrast (pun intended), Aria is much better.
- One of my vacuum backs is probably broken, and came to me that way. No effect observed, and none expected. I have no concerns for RTSIII reliability.
I agree with your opinion about EOS non-L primes (I have four). The build quality is middling, and the tactile feedback is HORRIBLE. Blame the AF mechanism, since Canon FD was fine. I love C/Y for beautiful optics, manual control of focus, and full-metal build for most of the lenses in the line.
Finally, to consolidate your flash questions, I mix and match the TLA280 (3 pins), RTSIII (3 pins), TLA360 (5 pins), and Aria (5 pins). There is no difference in behavior for TTL use. I believe the 5-pin connector gives some automation with auto (non-TTL) mode and with N system cameras (perhaps auto-zooming). RTSIII preflash metering works with anything it can fire, including studio strobes.