If you want to keep the cost down then just drop in an RB25DET. This will have heaps of torque for your VK to get off and moving. Beware that if you want 300kW from the engine that although possible, it may be expensive. If you are looking at that sort of power then spend the extra up front and go RB26DETT as it will do 300kW easy. It also opens up the opportunity for greater power mods later on.
The best way to do it is to get an R33 half cut for about the $2500 to $3000 mark. That will get you a long way there. The rest is up to you. If you do most of the work yourself then you will save a lot. Otherwise bank on about $1000 t $1500 for a sparky to do the wiring. Allow about $500 to $700 for an engineer. Odd bits like VL fuel tank, fuel lines, crossmember, guages, etc... As well as what ever you have to do to the car to get it up to V8 spec - brakes, suspension, exhaust, etc... Allow another $500 to $1500 for odd bits like a little welding, air-con.
If you are starting off with a 4cyl VK, it may actually work out cheaper to look for a V8 one first as it will have the better brakes and suspension, as well as the 5 stud hubs. There will also be the issue with the engineering as well - going from a V8 powered vehicle to a turbo 6 is not a big deal, but to go from a 4 to turbo 6 is a different thing. You see if increse the power of your car by a certain percentage there is a lot more that needs to be certified by an engineer.