Jak Tesla:
I quite like the expensive old junker approach - something big and ill equipped as a result of it being fabulously ancient. A fixer upper, in other words.
When it comes to ship hulls, just about any Imperial ship could qualify as being fabulously ancient. :-) Some that spring to mind are the Murder*, Conquest and Turbulent hulls, plus the good old Sword has been around basically forever.
One thing I noticed when running Rogue Trader was that Profit Factor generally got neglected in favour of starting with a cool ship, so I wanted to make that process a bit easier. I also didn't have an archaeotech distinction.
* The Murder was the only ship hull I added. It fills the niche of a faster cruiser option, and I figure if there are stats in RT for traditionally-Chaos grand cruisers, that would be an obvious addition.