How much oil is there? My supra has an oily film\residue in the intake piping aswell and have been assured by other import owners\mechanics that it is normal for turbo cars with a fair bit of mileage. If there is heaps though it is most likely your turbo seals are gone and they will need rebuilding.
Does your car blow any smoke?
Gunmetal GTR's look tough as, congrats on the purchase. Regarding the turbo upgrade I thought twins wouldn't be that expensive because you could just bolt them onto the stock manifold?
Well if your definitely sure your not chasing any extra power the N\A Supra might be the go. They are a very comfortable and reliable cruiser. Only cons are limited boot space and a smaller backseat than a R32 or Silvia.
I'd go the 32gts-t. Even though N\A Supras are fairly quick for what they are, you will regret it after a few months when you want boost. Everyone I know with an N\A is saving for a turbo conversion lol.
Don't know about any special exhaust valve but I do know the HKS T04Z has an anti-surge housing or as some people call them "speed holes". Gives it a nice whistling sound like the T51R but actually harms spool up by as much as 500rpm.