There's a semi-official army list for Seige regiments online.http://www.forgeworld.co.uk/Downloads/Product/PDF/k/kreig.pdf
You should have no problems using it, and it's been updated to the standards of the most recent codex, but if you want something no one can complain about, here's my recomendations.
HQ: Company command squad, give 'em a master of Ordinance, and turn 2 of the squad's vets into a Mortar squad.
Troops: Veteran squads. These are your seige engineers. Shotguns (due to the close range enviroments they'll be in). Give them the Demolitions doctrine to give 'em melta bombs to break into the most hardenened bunker, tank, or monster. Their CCWs can be represented by shovels, picks, and axes.
Fast Attack: You want tanks? Go with Hellhound squadrons. Inferno cannons are great for dealing with hordes.
Heavy Support: Your meat and potatoes. Really you're spoiled for choice here. 1-2 Ordinance batteries, and a squadron of Leman Russes can't go wrong.