Dear Khan....well since i work for the group importer for BMW here..maybe i can help..
-first of all there is no point in importing ur M3 from the states as the vehicles are left hand drive there..and we obviously use the right hand versions. The vehicle can not be newer than 22months & older than 3 years.
-import duty on ur M3, if you import it from the US or UK will be 219% on petrol engines bigger than 1800cc..this will cover ur freight, federal taxes..duties etc. since ur car will be used..u will get a rebate of 1% per month. so if ur car is 2007 model year..u will get a 24% rebate on the landed cost.
-ideally you should import from the UK as it will be a right hand version..also as you are aware there is a huge recession which has effected car prices all over..so you should be able to find a great deal there...at the end of the day..the M3 imported from UK will end up cheaper even after u sell urs in the US.
- As far as roads & traffic is concerned here in lahore
if you are a petrol head..it really shouldn't matter then..after all ppl here are driving porches, ferraris, aston martins, lamborghinis, corvettes...they seem pretty happy...if you love ur M3 enough...it shouldn't matter to u as long as u are behind its wheel.
-Dont worry abt the maintenance as we have established the BMW brand in all major cities mainly, lahore, karachi, islamabad and faislabad..and we have already imported 2 M5's to Pakistan aswell so we can definitley take care of ur M3...Also no one has imported the new M3 yet the new one that is..so it will be nice to see one drivning around in lahore. prepare to pick up lots of chicitas 
hope i was helpful enough. -