I might be running into a similar situation myself as the GM for my BC group. I've recently encouraged the party's spokesman (an Iron Warriors CSM Champion) and default "leader" to try and develop his character into a full blown War Smith. He's improved his Trade: Armourer skill a few notches already, as well as taken the Armour Talent; it seemed like a natural development to have him evolve into a War Smith type of Warlord with his background and goals.
Between that PC and the party's Word Bearer Sorcerer (who's going a more utility/skill based route to conduct rituals and the like) I feel that the party themselves will be able to oversee and conduct everything involved in creating a Daemon engine for their own use. As the GM, I've already thrown out a Magos NPC (or two) from the Hollow to introduce the warring Forges to the party. The party was pretty keen to make nice with them, so after the current compact I foresee a visit to one of the two Mechanicum Forges.
While I'm kind of directing my party into a mutually beneficial alliance with one of the two, your particular party can just feel free to raid the Hollow out for the designs they'll need to construct something like an engine. I'm sure the Magos have designs like that stashed away somewhere in their Manufactorums. Your party seems to be well ahead of the curve compared to mine on actually manufacturing one of these things; you got a few motivated Hereteks and the whole industry of their captured planet to back up their ambitions. It's a really cool idea. A Daemon Engine is almost like a War Band status symbol to me, like the group's flaunting off their swag for all the other War Bands in the Vortex to take notice. lol.
It's hard to say what "grade" of daemon you'd need to bind for something like this. Fluff wise I think it's not been touched on a whole lot, save for the references I put in up top to the Heart of Blood and the Omphalos Daemonium. Both were bound into Daemon Engines and put to use by the Iron Warriors. From the Lexicanum entries, one was stated to be a Daemon Prince. The other was a "favored avatar" of Khorne. To add into the equation, one was used to power a fortress's force field, and that the other was used to power a "War Chariot" for a War Smith. Binding a rival Warband's Daemon Prince leader into your group's personal Decimator Engine seems like a fun idea, now doesn't it? *Insert Evil Laugh Here*