You will go thru about 55 litres of petrol (a tank) in 300 - 350 km if you drive normally or 400 - 450 km if you drive mostly on the highway.
An R32 does not consume much less fuel than an R33. The difference really is only 500cc of displacement, of which you probably make up by being more of a leadfoot.
For 15k you are really in R32 GTS-t territory, any R33's you can find for that much will be auto, non turbo, old and busted, or all of the above. You will be stretching it for a good R32 GTS-t aswell at 15k
Id say get together 18k and get a nice R32 or R32 with some minor mods, exhaust, performance panel filter, maybe an SAFC or an EBC.