I’m trying to lay out a campaign arc for a future game. Generally, I don’t use pre-made adventures but a couple of the DH ones are really interesting so I’m planning to incorporate a few of them. I’d like some opinions of those who have run it on the following questions:

I have six players who are pretty smart and tough in almost every game we run together. We’re not playing DH now so I don’t have career breakdowns. But I’m looking for how to challenge them. Also, I want to incorporate the following adventures into the campaign that I have planned. It has a few one-offs but also an overarching pattern of two separate plots (actually one big one). I’m hoping that the big plot would continue into Ascension. In fact, the last two or three missions of my arc, I’m hoping will carry them to Ascension. So I don’t want to put them into Legacy and have them done (Rank 9) at the end of it.

Last Adventure of Purge the Unclean (#3)
-What rank would be appropriate for this game (6 players).

House of Dust and Ash
-What rank would be appropriate for this game (6 players).

Haarlock’s Legacy
-What rank would be appropriate for this game (6 players).
-How much XP should I estimate that they’ll receive by the end?

Basically, I want to be sure and challenge them but also space things out. I’d like to Dust/Ash a mission or two before Legacy. Also, the last 4-5 missions as Acolytes I want to be part of my campaign arc.

All of the missions you have mentioned are built to be a good challenge to characters in the 4-6 rank range. The game generally assumes a well rounded and skilled group of 4 characters, so you will want to tailor certain encounters to your group depending on how lethal they are. If your six consist of two Guardsmen, an Assassin, an Arbitrator, a militant Cleric and a Psyker then you will obviously want to add more mooks to the fights and perhaps scale-up the big-baddies. If they are instead more cerebral and social, packing a few compact stubbers beneath their robes then perhaps remove a few of the mooks?

The nice thing about this game is it is far more important to have a well-rounded team than to pile on sheer firepower. If you don't have a knowledge-monkey, a face-man and a skilled investigator in the mix then the murder-machines will not often know what "needs killin'", and that leads to failure, which is bad... A skilled medic is likewise a gift from the Emperor! The other nice thing about DH is that even the "squishy support careers" can get pretty bad-ass if they play things smart. Case in point: Our team's Adept is surprisingly skilled with a sword, especially for someone we hired for her brains!