"Average" purchasers (i.e. the ones the Blue Book are meant to cover) don't care about the rarity of a car when making a buying decision. For your average car-buyer, a V6 auto anything is worth more than a 1.8T manual anything... Certainly an enthusiast might pay a little more for the car, but be prepared to wait a while for such a buyer.
But in all honesty, we aren't talking about a whole lot of money either way... it's a 10-yr-old non-luxury car with nearly 1/4 million miles on it, from a brand that does not exactly have a reputation for rock-solid reliability. No matter how you slice it, it ain't worth much. (For reference, Edmunds says it has a private-party sale value of $2,417, assuming "good" condition, which a car with a bad oil cooler isn't.​) Yes, everybody here feels the B5 was a very special car (because it is/was), but that's not worth very much for a car so young, especially since there are plenty of MUCH lower-mileage examples of the car out there. If I was shopping for a 1.8T M/T 4Mo, I'd bide my time waiting for a lower-mileage example to come on the market unless you were offering a very good deal.
And you'll have a hard time selling it for much more than scrap if you don't fix that oil cooler. While an enthusiast might guess that the coolant/oil contamination is likely just an oil cooler, anybody else (including most mechanics) are just going to assume it's a head-gasket, and therefore Certain Doom for the car.