Ok, probably opening a huge can of worms here, but here goes nothing.
The 'options' section for penitent engines contains the following "Include an additional two penitent engines....85points per model".
So my question is this - do you have to buy two additional engines or can you buy just one?
The argument for two is: the wording is such that it does not give you an option to buy one. If it was meant to, it would say, as the other unit entries do, 'include up to x pentient engines'. The wording seems quite deliberately chosen to mean you buy 2 extra or none.
The argument for one or two is: a) if you had to buy 2, why give the points cost per model rather than just a total for both? b) why on earth would GW want to force you to buy two models rather than just one (other than of course for financial gain)?
To me, this seems to be a case of badly worded rules (and as I have mentioned elsewhere, GW does not proof read WD) and the only sensible thing is that you can buy one or two additional engines. However, if we play RAW, you have to buy 2 or nothing....
Have at it.
Edit: Rules on p91 of WD381 for anyone who wants to look them up.