Converting to the RB25DET is much more viable if you are on a budget, as it's close to pull out and drop in, once it's done you will have little trouble with issues changing a motor, I've been pulled over multiply times and police have never noticed the motor difference between the 20 and 25. In the case since it's a newer motor a few bit's of paper work need to be done, this is based on my experience a few years ago with a yellow sticker RB20 R32 running a RB25 without papers and having to update the vehicle. If you want to complete the 26 conversion I'd budget near the $10k mark once you changed your gearbox, clutch and all minor bits and pieces. RB30 idea running a 25 head is viable, again once all the hands and exchanges have taken places with parts and servicing your $50 block will be near $5000 once you can drive out, assuming you are going to a run a stock turbo, beyond that the figures jump up. I'd suggest getting advice from people who have completed conversions and what sort of price they have payed, also figure out what sort of power and delivery you want. More to the point, my friend completed the RB26 swap for his R32 about 4 months ago without any issues, after all costs the figure was close to what I mentioned earlier, with cheap parts and good value labour from a local workshop and is still registered without any issues and has been pulled over several times by local police. Simply avoid the big single blowers and you will be fine