Celebrim, you're slipping....
U1 does indeed have poisonous monsters, but they are not overly punishing, as their poison is not 'save or die'. Plus there is a Potion with 2 doses of Neutralise Poison very early on in the adventure. The Assassin is dangerous, there is some Yellow Mold to catch anyone not being careful, and Rot Grubs which will challenge them if they are impulsive - but beyond that the adventure starts off very simply, and very steadily. If the party are cautious they should be fine.
And I disagree with U2 - yes it can be over very quickly, but in my experience most groups make some early inroads before realising their mistake, leading to some interesting roleplaying (wriggling!).
L1 and L2 are not good starting points for beginning DMs - L1 is very sandboxy and requires a far bit of work from the DM - yes the material is good, but its organisation is horrible, and the OP has stated she is short on prep time. L2 is complex to run, requires multiple readings and re-readings of the adventure, and should only be attempted by a DM who is confident and who again has plenty of time to prepare. Both are worthy adventures, but really not ideal in this instance.
The main reasons for suggesting U1 - it's easy to digest, is logically presented, has a great plot, is initially very forgiving, and breaks itself very nicely into chapters. I've run it twice in the last 12 months, and on both occasions the parties did it in 3 sessions of 3-4 hours - session 1 explore the upper floors, session 2 go into the cellar and below, and session 3 the fantastic finale on the ship. This finale is tough (1 party succeeded, the other failed dismally) but the House/Cellar/Caves provide just about the right amount of xp and treasure to get them to 2nd level and therefore give them a chance of success.
I agree with UK1, I3, I4 and I5 though - very good and very diverse, but let them play and enjoy lower levels first in order to get accustomed to the game. The group I'm running at the moment have played UK5, T1, UK2, UK3, UK4, UK1, I1 and C1, and have just started I3. It has made for a very enjoyable campaign. Sadly I couldn't start them on U1 or N1 because one of the group had played them before, and UK5 is somewhat mediocre, but other than that all the adventures have been top drawer. UK2 and UK3 are under-appreciated gems.
The Dragonlance series should be burned on sight.