Some items will be dropped on the ground near your party, some stay on you, and others will be found soon after you finish the first dungeon. (I've only played Siege once, so I don't recall exactly how it all works -- I just remember I did get everything back eventually.)
Imoen gets removed from your party at the beginning of SoD for "story reasons." (Debatable whether it's GOOD story reasons, or just... "reasons"...)
If you haven't dual classed her to a mage by that point, she'll do so on her own. While I don't agree with a lot of the things Beamdog did with SoD, this one does admittedly hold some degree of consistency. Baldur's Gate 2 starts with her already being a Thief (7) / Mage (8). That was a design decision by Bioware way back in the day, and cannot be changed now. So if you haven't dual classed her by the end of BG1, SoD would admittedly have to be the time when that happened in order for everything to sync up right when you get to BG2.
After you complete the starting dungeon in SoD, you'll soon after find a chest which contains any gear from characters who didn't get included in your starting party (namely, Imoen).
If I recall, any bag/case/container items (Bag of Holding, Scroll Case, etc.) won't import into SoD. Those types of items were not in the original game, and were added in via mods or the EE's many years later in a very hacky way (basically, those containers summon an invisible shop keeper who will buy and sell items for no cost). So, before importing into SoD you should empty all your containers and either sell the junk or put the good stuff in your inventory.
Yes, it would be keen if the game actually tells you that upon importing (i.e. a dialog which pops up saying "WARNING: Container items cannot be imported. Move their contents into your inventory first, otherwise any items within will be lost upon import.").
Oh, and to address your very first statement: yes, when you hit BG2 all the NPCs get reset to a default state. Some of them do have slightly improved stats over their BG1 versions. Jaheira gets +2 DEX. Viconia gets +3 WIS. Edwin gets +1 STR (umm... okay...) and +1 WIS. Minsc gets +1 DEX and +1 CON. But they do not keep the spells, weapon proficiencies or other attribute bonuses (i.e. tomes) you may have given them during BG1. If you want to completely integrate all the games together and have your NPCs retain their stats from one game to the next, check out the Enhanced Edition Trilogy mod -- which merges all the games together into one uber-game installation.
Post edited August 09, 2018 by Ryan333
