Using the video card as an analogy, imagine you have an old PCI video card and motherboard. Thats your 18RC. Upgrading to a better carby engine like an 18RG is simply buying a better PCI video card and swapping the cards. Simple work.
A 1GGTE or 1JZGTE would be equivalent to basically buying a new AGP or PCI-X video card, then having to get a whole new motherboard (ie loom + ECU) as well, as well as probably upgrading PSU, RAM etc as well to get the most out of it. Not simple at all, especially for someone who is not an expert in the area. Best bet is to buy the components yourself and then give them all to a professional to build.
Yeah... that's pretty much the plan. Find out all the bits and pieces that i need for the conversion, purchase them and then hand the new components off to a pro with my car otherwise all ready to be converted. ie, the bits that need pulling out of my car would be already taken out by me at this point so ideally all that has to be done is that the pros in question just install the new components.