Descent into Avernus. What level to start at?

Hi, I'm going to be running Descent into Avernus later in the year. Been reading through the book and looking at reviews online. A lot reviews said that the start can be quite tough and having players at a higher level than it says in the book. 

The book starts the players at level 1. I was thinking of running a one shot set in Baulders gate before running the campaign so the players can level up a bit. 

What level would people recommend for players to be at for Descent into Avernus?

I know the encounters are meant to be hard but I don't want to TPK my party. Its not the sort of game style we play. 

I don't think you'll have any issues if you give your players a bit of a boost before they get into things. Honestly, the early sections of the adventure are the toughest, particularly that initial dungeon - I ran it at level three, and it was still a challenge, albeit an achievable one.

Personally, I would start at level one and just be a little more generous with their advancement, or just start at level two. Either way, aim for them to hit level three around the Elfsong Tavern, and go gently with the following dungeon - but honestly, as long as you make sure not to deviate wildly from the recommended levels, you should be fine whatever you decide.

The Elfsong is the first encounter in Descent, yes?  So I'm not sure what starting at level one and being generous with advancement or starting at level two means if you want them to be level three by the Elfsong?

I agree that level three is a much safer level to try the Elfsong and Sewer Dungeon.  We had a party member downed in basically each of our early sessions.  Lots of hiding and recovering (which, since they never patrol anything, works fine).

If you're using 5.5e, it kinda gives you an extra level to start with, though, so I'd take that into account.

Yeahhh, in retrospect, I wasn't very clear with that second paragraph. That's my bad.

To clarify, "be generous with their advancement," was intended to be general advice for the adventure as a whole. Being level three for the sewer dungeon (whether they achieve it via one-shot or just starting at level two) was a separate recommendation.