1) what they have is limited only by your imagination. As I recall, the first Soul Drinkers book opens with a Marine assault upon a large space station owned by a Rogue Traders, so... why not a mobile version of the same? In Matthew Farrer's Crossfire, one Rogue Trader fleet is practically an armada to itself. I don't imagine that the ship would be anywhere near as well-armed as such mobile fortress-monasteries as you named, but sure, there's nothing to stop a Rogue Trader from sticking engines onto a large hunk of rock, or some such.
2) well, it's certainly old, given that the Mars class is its replacement. I dunno if it has been positively identified as a HH-era ship, but it's probably a safer bet than most other BFG ship variants. As a minor downside, the moniker "Exorcist" sounds like a name for a class of ship more grounded in the modern 40k mentality, accepting daemons, exorcisms, etc. I imagine shipwrights in the age of the Imperial Truth would think long and hard before naming a class as such...
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!
Last edited by Mossy Toes; 08-20-13 at 05:44 AM.