A 2003 3.4 Surf sells between 30-32 at a very good condition (SSR-X). One can get a lessor price too, if searched hard.
Anything more than that is rip-off for that vehicle. Yes a later model can fetch a higher price.
The mileage /100kms averages to about 6km/ltr ..... It all depends on how you drive.
As for a Marks on a double cabin, remember that your rear cab will always be cutoff from the front cab. I don't know how the cooling and everything works in the Marks.
The Indus Hilux does not have the rear sliding screen to allow access to the pick-up area.
The Indus Hilux is a 2.5 Turbo Diesel, whereas the Vigo has an Intercooler Turbo Diesel, increasing the power by 35%.
Thing over your needs very carefully and then invest, you don't want to get into a distress sale later on.