In the early to mid '80s, conversion was often a good deal. You could get potentially better models than were being offered in the US, often for much less money in the end. The dollar was relatively stronger then, and the regulations were softer. Also, European cars in the US were much more expensive in real adjusted dollars back then. They were going for lower volume/fatter margins as a business model in the US, while doing a higher volume/lower margin strategy in Europe. My 300E was $40k in 1986 dollars. That would be $64k today!
I cannot imagine it being worth the bother or expense now.
Even your 1991 model would likely have to meet tough 2003 standards on every front.
We had a Grey Market 1981 BMW 745i in the family.
Sweet machine. 240hp turbocharged, ABS (before this was even available in US), no cat, thin bumpers. A kneck-snapping rocketship when the big turbo abruptly kicked in.
Always a pain getting it to pass annual state emissions test.
Had to tweak the mixture lean, then set it back after the test.
Also, the cooling system was not up to even our New England summers. Clearly designed for moderate German climate.
Mailorder parts were surprisingly easy to come by, even in the pre-internet days.
1986 300E 5-Speed 240k mi.