Right, since it seems like no real consensus was reached, I guess I'll have to sort stuff out after all.

Armies should be built using your Codex (if you have one) or your Index if not. Forge World is fine if it has an official 8th edition set of rules.
Build a list for either team play at 750 points (max 1 detachment) or solo play at 1500 points. (max 2 detachments).
Missions should be rolled at random from the rulebook. (That's still a thing, right?) If either player has already played the mission rolled, you may reroll it once and once only.
As a rule of thumb, bring an army that will be fun to play against, not one that is meant to table your opponent in the minimum time possible.
Games will be limited to 2.5 hours, with a little leeway for finishing turns.