theres no real answer. it depends on your bodywork, your wheels, your tyres, ect ect.
on the factory R34 GTT youd get away looking real low with 18-20" rims and only slightly lowered.
on the other hand someone like myself with low/wide bodykits....19" rims still make the car look stupid high.
josh, the bomex is also a pretty big bodykit, so the car will look very veyr low by dropping it.
here are a few pics of bomex kits.
as you can see, the cars running factory rims (17") and no lowering...and the car is scraping on its ass.
so keep that in mind aswell.
best bet with a kit like the bomex is prob 19" wheels, some minor lowering...and you should be sweet.
but 18s can work aswell. take a photo of the car and get someone to chop it....if you havnt tried already