Sisters Of battle Exorcist Kit Bash

I have signed up to the Bolter and Chainsword 2013 painting challenge. I committed to 5 converted sisters of battle and two Exorcist tanks. I have always loved the Prioris pattern tank, it's highly detailed gothic design is both attractive and daunting at the same time. Years ago I looked at both the forgeworld Sanctorum pattern and the GW Prioris pattern. I evaluated them on difficulty of build, difficulty of paint and practical,it's for gaming. The Sanctorum pattern is a Rhino Mk IIc kit with a resin turret ring and the distinctive "Coke Can" missile launchers. It is made up of four peices and looks OK, not great, just OK. Assembly is simple, and they are not particularly heavily ornamented, the Rhino Chasis can have as much Or as little Sisters Bling as you have in your bitz box, but is supplied as a vanilla Rhino otherwise. It is also highly practical to transport and game with, being essentially a plastic rhino, a shape for which many companies provide carry cases.
The Prioris pattern from GW uses the Sisters variant Rhino IIc with the castle. Then it is modified still further with pewter inserts modelling the launcher bay, pewter castle raising the back of the vehicle as a base for the pewter pipe organ. Not to mention the pewter glacis plate and side armour.
As a model, it weighs a ton. It is highly impractical to carry about due to the crazy shape. The detailing stunning, and there is no mistaking this vehicle as a sisters vehicle.
Back then the decision I took was to go with the Forge world model. Practicality winning out over design. This time I want to use the Exorcist as a basis for a conversion. It will be based on the Immolator turret with Valkyrie missile pods instead of flamers. To keep the theme with the Prioris pattern it will have the pipe organ miniaturised and placed behind the pilot.

As with previous planning posts like the Bane Wolf Squadron and the Valdor Heavy Tank Destroyer, I will be looking to provide a short background and look for some context for the model. By doing this I hope to:
- give some direction to the build
- add some flavour to the model.

Like before this is done by asking three questions:
- What is an Exorcist;
- Why does the Priory have one?;
- When are these particular Exorcists depicted.

What is an exorcist?

From the Games Workshop website:
Sisters of Battle Exorcist
The Exorcist is only vaguely understood by the Tech-priests who must service it. Each one is an individual work of art, its battlefield role of providing devastating long-range fire support being secondary to its significance as divine symbol of the power of the Emperor.

From Lexicanum:
Besides the Rhino, Exorcists are the oldest vehicle in service to the Adepta Sororitas.
The first Exorcists were produced in the Age of Apostasy on Mars, of which some from that period, known as 'Prioris' pattern, still survive.

These machines are highly venerated, although their Machine Spirits are known to be temperamental and require constant work and prayer.
Each vehicle is regarded as a work of art, as much a symbol of the Emperor as a provider of long-range fire support. While other patterns exist, such as the 'Sanctorum', they are not held in as high esteem as the original vehicles, blessed from the nearby divine presence of the Emperor when they were constructed.

The ancient, arcane technology of the Exorcist launcher is only vaguely understood by the Techpriests of the Adeptus Mechanicus. The missiles themselves are armour-piercing explosives and capable of annihilating enemy tanks or destroying squads of heavy infantry in a single salvo assuming the launcher does not malfunction. Though potent, to the Sisterhood its primary purpose is to serve as a divine symbol of the power and glory of the Ecclesiarchy and serve as a mobile shrine for the Sisters of Battle. Each one is hand-crafted and unique. They may also be upgraded with Blessed Ammunition, Dozer Blades, Extra Armour, a Holy Icon, a Hunter-killer Missile, Laud Hailers, Pintle-mounted Storm Bolter, a Searchlight and Smoke Launchers.

Why do the Ark Angels have one?

A prosaic reason really, the codex has too few other options. A fluffy reason would be that the high grandees of the eclesiarchy only supply the convents with one vehicle. When the Ark Angels were invested by the Comvent Prioris, three ancient 'Prioris' pattern hulks were taken from stasis, to be reconsecrated to the God Emperor and pressed once again back into His service.