Interesting idea, I had the same idea some time ago but in the end just plumped for a razorback!
At the time I was thinking of something along the lines of a Predator, based on the old RT rhino chassis. The top plate would be turned around, so the hole for the turret would be at the back, and their would be no sponsor weapons.
I kind of imagined it to have a real-life equivalent of the Israeli Merkava tank, which has a turret and limited troop carrying capacity - essentially you are giving troops the possibilty to deploy into very dangerous areas, back it up with far more firepower than nromally held by an APC, and also provide protection against lower-powered anti-armour weapons.
So, in a nutshell this is describing the Razorback - one of my main beefs with 40k background is how it supposedly took 5000 years after the Heresy, and even then someone to stumble upon 'lost plans', before anyone came up with the idea of putting a gun on the roof of a rhino - presumably those plans looked something like this:

I would say the hotbed of military technological development presented by the Great Crusade would have made it almost certain that a vehicle with the characteristics you mentioned would have been developed - after all, you can imagine it being perfect at being used on planets brought into compliance but still exhibiting rebellious tendancies from some elements of its population. Arguments concerning politics aside, this is exactly why and how the Merkava was developed by Israel and is a continued success in its role.
As for rules, I guess being conservative with it you could simply say its a 'counts as' razorback. Otherwise, perhaps add 15-20pts for the additional armour of a predator? I wouldn't add too many points, as moving forward at full speed to drop off troops will negate the use of its weapon.
Hope this helps, those are just my thoughts on the topic!