So I have heard that flushing out high mileage transmissions is not wise and may or may not hurt it more. That makes me wonder about getting a few mods: shift kit/shift improver, torque converter, and corvette servo.
Could I do all of this, or should I only be limited to 1 or 2?

I suggest you do all 3. As far as the converter, the choice should depend on your future mods, but if you're still running your stock 2.73:1 rear axle ratio, you'll find anything looser than a 2000-rpm-stall is annoying in town. You should start with a 3.42:1 rear gear, then get a 2500-stall converter, like from TCI.

yes it's VERY annoying, but the gear upgrade is planned to be really later.
what about the fluid for the converter though?