I'll take 10-14 hour trips in my car occasionally. If you get an e46 with the 15 or 16" rims, and standard suspension then you should be fine. I can cruise at 80mph and feel totally fine for hours. The lower profile tires you get, and sport suspension will make any car a little harder on long trips.
You should get great gas mileage with the 320i too, which is a plus on long trips. I'll average 32 mpg on the highway going about 70 mph in my 323i(5-speed trans)
I've kept debating getting some BBS Anthracite CKs and a Hotchkis suspension kit with Bilstein struts, but I really don't want to kill the long-distance driveability of my car. Oh well
Live slow, die whenever.