Good luck with the build
I've had the privilege of driving a 2.0TS with a TD04HL-16T turbo from a Volvo T5 for a year. It would start boosting at 2500RPM with full boost at 3000RPM and drop off just before 6000RPM. I am using the same turbo now in my new 2.0TS build.
1st off you'll have to say which SPECIFIC Garrett T25 you are going to use so to know which map to look at. T25 only refers to the Turbine, what about compressor?
Use this wizard to help:
Garrett Boost Adviser - Online | Turbobygarrett
The GT2252 / GT2259 are closer matches to the one I had, the GT25 will spool at a bit higher revs than the TD04HL
Turbine TD04HL : 1.80" Exducer ; 2.05" Inducer ; 77 Trim
Compressor 16T : 1.713" Inducer ; 2.205" Exducer ; 60 Trim
EDITED: Then you talk about the stock manifold with no cats from the old TS motor... You recon the turbo will fit OK? If you refer to the 16V 1.9 diesel manifold, it could be an easier fit?
You only need to run rich when you are in boost, not small throttle (out of boost). So yes your trick of no lambda will work, but it will stuff up your consumption royally. Good for track day car, but not daily driver.
On the oil - yes keep using the same stuff, just replace quicker than normal service intervals