The nice thing about playing BG1 is that you can import your character with the exact stats from the first game into the second, at whichever level he is, and with any permanent stat bonuses he may have picked up in the first game, instead of starting with a brand new level 8 character.
It's very satisfying to do a full playthrough, starting at level 1 from the first game, all the way to the end of the BG2 expansion Throne of Bhaal with the same character.
You don't really need to play it to understand the story of BG2 but it definitely helps. Especially when you run into old companions.
If you get both BG and BG2 (which I can recommend, they're both great games), get tutu. It allows you to play BG on the BG2 engine, which helps the game.
Definitely. It also allows you to use the BG1NPC mod which adds more conversations between party members, which is something Bioware did a lot of in BG2 but very little in BG1.