Turbo oiling -- the oil source will be more difficult to solve than the oil return. The dipstick might be a possible location, but you wanted to put the turbo on the opposite corner of the engine.

Fuel mods -- a lot of research is required here. Not much is published on how much fuel the system can deliver. You have to make sure you don't introduce more air because the results won't be cheap to fix.

Timing mods -- If your engine has a knock sensor, you can probably leave the timing alone.

Most internal wastegates can be modified to open later but not sooner. An external wastegate can make up for that. Fortunately, air is metered after the turbo on a KE system so you don't have to worry about recirculating 'wastegated' charge.

I saw a picture of the 420 exhaust system at timevalve.com. That is one ugly precat. I agree that a turbo should take its place

Have you considered replacing your air pump with a Paxton or Vortech supercharger?

Sixto
91 300SE
81 300SD