rw,
1. I wouldn't install a new cartridge in an arm I knew was not positioned correctly. That will make installation alignment difficult if not impossible. So you will not experience the potential of the new cartridge.
2. I've owned a couple of SOTAs and they can be very good tables. But as others mentioned they do require maintenance (more than just bearing oil). As I remember, the MDC-800 was a decent arm so probably doesn't need to be replaced, once installed correctly, so long as it is not damaged.
3. I prefer the stability of speed in a decent DD table, such as the new Technics. So my choice would be to buy that. And there should be a buyer for the SOTA, particularly someone who can upgrade it themselves for a reasonable cost.