I'm not entirely sure on the recommended level, I know that there is a way to go from the IWD areas to HoW without completing the former (then you can return once you've finished the expansion). I know that it's possible to do that, but I haven't personally tried it, I always complete the IWD areas then go onto HoW.
That transition should be flawless, all characters are raised from the dead (if killed in the final battle) and have all their equipment. To get into the TotL areas you only need to talk to a halfling in Lonelywood (I think the ecommended level is 15 for those areas) and again all equipment is automatically transferred along with your characters both going into and exiting the TotL areas.
No importing/exporting is needed, the process is automatic.
