an interesting subject that seems to scare most... while others throw around phrases like "whatever money it takes"...
Being in export/import business... and not having a need for a diesel (yet), all I can say (speaking from personal experience) - bring a rig here, yes - registered from abroad, dont need to bring it here for "keeping" - just as a rig for travel.
So, your rig that you bring from abroad cannot be "legally" registered with your local DMV, being that you havent gone through all of the DOT Approval loops...
Thats why it become simple at the point where you go & purchase a salvaged one from an auction.
Swap out all the major parts which carry VIN info in order for the DMV to "re-issue" title in your name for the salvaged rig.
So in turn what you get is an exported rig from Europe, with a factory installed diesel, everything working as it should, legally titled & registered in US.
Costs??? - all depends what & where you buy...
Time??? - shipping from Europe to US about 2-4 weeks (depending whos brokering & loading the rig)
Registration fears ??? - did you steal it? No, so why would anyone "dig" too deep thinking its stolen if its not, or any cop care what really is under your hood???
disclosure - this will work in most states, while some places that are A** about smog tests & other issues will not work, but so as any other CONVERSION will not work and wont permit you to register your rig with DMV, so - think more broadly... learn the local & other state rules & laws that apply to registrations & other processes...
Hope I wasnt too confusing...
p.s. for anyone thinking otherwise, consider this as a fact of my personal experience... if you believe & think otherwise, thats great, I'm happy for ya.