Let's say that, for whatever reason, an Explorer wishes to acquire a Penitent Engine.

This is certainly not for the purposes of using tech-heresy to equip it wih autocannons and then wiring the Explorator into it, no sir. Just for research and punishing heretics. You know those dastardly heretics. They need to be taught a good lesson.

I think there are stats for these machines in one of the Dark Heresy supplements, but I don't think it has an availability. Any ideas for what to make it, and any additional issues the process would have?

I would suggest not letting it be aquired by normal profit factor tests, this is a most holy artefact dear to the ecclesiarchy and one that may not be used heretically, the church would make sure that one is only given to one they feel they can trust. I would make aquiring it a lesser endeavour (Creed). 

Objective 1: Would be to make good contact with the Ecclesiarchy and start up negotiations over the acquiring of one, this objective doesn't deal with actually getting it, just opening up the negotiations and proving to be good Imperial citizens as well as righteous men of action should be required, after that the Ecclesiarchy can test them, ask them to help them or ask for donations. 50 Achievent points

Then you can add some extra objectives from what you feel, you should have more of these than needed to complete the endeavour so that they can choose how to get the help from the ecclesiarchy, I'll give some examples here:

• Aid the Righteous: Several pilgrim colonies and chapels exist in the Koronous Expanse and many are in dire need of donations and help. The Ecclesiarchy has been contacted by one but have been unable to aid them yet, if the Explorers can travel to that world and rebuild the chapel/cathedral/colony and give them resources (from their own stock as a proof of devotion) then they show themselves as blessed. Make this a multi-objective (part-endeavour) which is made from some parts, aquiring equipment and competent personell, travelling there and fighting a small band of bandits or xenos that threaten it. 500-600 Achievement points.
• Kill the Heretic: An agitator speaks against the church and his elusiveness have made him escape their clutches many times, the explorers are asked to bring him to justice. 150 Achievent points.
• Test of Faith: Some ancient rite supposedly made to test a persons righteousness is suggested as a way for the explorers to prove themselves. 10-300 achievement points depending on how dangerous you wish to make it.

After that the last objective is closing the deal, writing the contracts, speaking the oaths, making the trade. This is an easy objective (10 Achievement points) and is really just there for closure, that said they could fail in this step, an explorer refuses to speak the oaths or pulls out when the price becomes too high.

It can of course be stolen or bought on the black market but I'd probably make that a lesser endeavour as well.

Leogun_91 said:

I disagree. Profit Factor includes such elements as trust and favors as much as it includes material assets.

Step two: get yourself an ecclesiarcal army including penitent engines, repentia sisters, and arco flaggelents.

Step three: Profit.

Step two: begin supplying your ecclesiarchal friends with muderous rampage engines of penance, but keep the "surplus" for yourself.

Step three: profit… In several ways.

Wouldn't it be easier just to get a Sentinel?

The penitent engine is, fluff-wise, exlusively the province of the Ecclesiarchy. If you were to acquire one clandestinely, I'd put the availability at Near Unique owing to the difficulties of acquiring such a fiercely protected construct. Also, don't ever let any Ecclesiarchs catch you with it, or you'll have a battalion of Battle Sisters up your fundament before you can say throne blind me!

Yes, Penitent Engines are property of the Ecclesiarchy. But even if the group doesn't have a missionary, you ship will most probably have priests and a chapel. So you don't acquire the engine for yourself, but for the Ecclesiarchy delegation upon your ship. Problem solved. ^^