hey mate,
i dunno why you would pay anyone anything for this job, its simple.
Run the wires through the same way you wud run a power wire for your amp, if your not sure on this root in a r34 look in the car audio section of these forums and there is a guide.
The way is through the top corner of the drivers footwell, then you remove the wheel inner gaurd(or just the rear part) and it will follow through to the engine bay.
With the gauges they will all have fittings/adaptors which the gauge box, should have instructions on where they should be fitted.
in answer to your question re the oil filter, i would play it safe and drain all the oil into a clean container, and re fill it or even add another oil change in with the huge amount of money your saving by d.i.y !
Im honestly not sure how you would consider paying 300-600 for a job that should take an absolute noob like myself an afternoon at the most.