I've converted B10 to 5e and run it. It's a ridiculously fun adventure. Most monsters convert just fine. A few require a bit of extra work, but not many.
Depending on the number of PCs, you could start with PCs of any level from 1-3. Though I wouldn't have all of them be 1st level. The only issue you might have is that the adventure is worth lots of XP. I'm missing a few sections on the spreadsheet I just looked at for my conversion, but there is easily over 60,000 xp if you clear everything (at least using the xp from the monsters I used in the conversion).
When I run it again I'll either reduce monsters/excise some encounters, go with milestone leveling, reduce the xp for the monsters to slow leveling (say, 2/3 or so), or just let the PCs hit 5th level earlier than I would like.
But it is a really fun adventure.
I've run U1, N1, then B10. U1 was fun.
The PCs liked the haunted house/ship as a change of pace from a typical dungeon. In my version, they didn't get a ship in the end but they did get access to a ship in the future (and they got plot hooks for X1!!).
N1, which I've run twice (well, halfway through the second run), was a hit because of how deadly it was. First time, a PC died vs. the BBEG in the dungeon portion. I had a good time hamming it up as the villagers (either mind controlled or not). I even managed to cry as the rescued girl in the Temple. The look on the PCs faces when the DM cries and begs the PCs to help is one of the highlights of playing 30+ years of RPGs. And the BBEG in the Temple (which party 2 finished yesterday) brought a cheer when he died yesterday. He one shot two PCs (who thankfully didn't die outright) and there was actual fear at the table. And yet they weren't mad at me because the module does a good job of preparing the PCs that evil forces are at work.
But nothing... nothing... prepared the PCs (of the first party I ran through it. Should get party two started next week) for the brutality of B10. The look on their faces when you tell them the burning farmstead they are in is under attack by 40 goblins is priceless. Sure, the adventure is basically a series of MacGuffins of "rescue this, rescue that" but they were under a relentless time pressure to save the NPC (whom they had met and befriended as 0 xp characters in the generic starting inn) and then find the Lost Valley. It's also quite easy to tie finding the Lost Valley to some grander campaign versus some awful BBEG.
Oh, and one of the PCs decided his background was slave. So of course the slavers in B10 were those who had enslaved him. The PCs viscerally hated team bad guy. They wanted Golthar and his entire evil band destroyed. I added as many NPCs the PCs had met up to that point as victims in one way or another.
The adventure is just so long it feels oppressive. By the time they hit the Lost Valley they really, really had no patience for the pettiness of the inhabitants.
