Running an AD&D adventure ("The Ghost of Harrow Hill") in 5e; any advice?

[BisectedBrioche]

CR/XP values meant very different things in editions before 5e, especially relatively to what a PC level means.

I advise you to check each encounter in an online CR calculator. Not that CR is the end of the reflection, as you stated for the Shadow.

Are the PCs going to be lvl 1?

A party of 4 level 3's (Bladesinger, two Warlocks and a Paladin).

To be honest, I feel like the idea for most of the encounters are to keep the PCs on edge while they explore the house (and burn up some of their resources before the boss fight, assuming they don't hit the fail state), so I'm OK eering on the lower side.