Any turbo reconditioner worth their name should be able to do the same.
When clocked correctly it will fit to your manifold and downpipe. You have to adjust the water lines as the 18g has 14mm connection rather than 12mm like the vf34.
You may need to shave a little off the block if using the larger shaped compressor like mine. You can get another type of 18g with a different shaped housing that flows the same (or only slightly less) that shouldn't need any modification to the block or head. Ryan probably knows the names of the different housings.
As mentioned, I'm yet to fit mine so can't be specific. Ceedubya has fitted his and mentioned nothing about shaving any metal from the engine.
Cost me about £800 all in from here -->
http://www.scoobyclinic.com/frameset.htm
I'd also look at Owens and other manufactures garrett/IHI hybrids if going down this route.