1. Mitsu TD05 Turbo is perfect for the 300E. Must be off of a Starion or Conquest Intercooled. Those were 2.6L units
2. RHB6 is too small. It will start boost early, put peter out by 4000 rpm.
3. I would expect the TD05 to boost about 1,800 rpm compared to 2,300 rpm of a Air Research T-3.
4. 9.2 to 1 compression. I would recommend 7 psi intercooled.
with 9.2 to 1 expect even a quicker spool, almost nil on the turbo lag!
5. Drag Strip...Run Unleaded Race Gas mixed with Premium Unleaded. Run about 20% Race Gas. 8 Psi should be fine.
I would not push the stock pistons past that. One step Colder Plug may also help. Nippondenso or Bosch Silvers.
6. Retard Box. YES! your compression is high. you want 25 degrees advance at FULL boost/3000 rpm. (16V engines can run 28 degrees, but yours is not)
7. Your stock rods can handle 75 HP per cylinder. The crank will handle 18 psi no problem. Cast Pistons can be run up to 11 to 1 compression (8 psi) Forged Pistons make too much noise. Detonation will break the best parts in the world! So avoid detonation.
8. My friend who is a MB mechanic and Auto Trans specialist told me the 300E transmission will handle 350hp no problem. There are some tricks inside that can be done for more HP. Upgraded Transcooler may be necessary. I have seen the trans handle that amount of power before so I think it will be no problem.
9. You forgot extra fuel. We can tackle that answer later.

GOOD LUCK!