Actually you can play them just as IF and still get a nice performance out of them!
IF perfected the art of siege warfare pioneered by the Iron Warriors. The
IW were founded earlier but the IF gave the concept a more dynamic feel.
In that view Whirlwinds and Vindicators are your friends, as well Thunderfire cannons available as squats on ebay - with a repaint, of course!
When writing your list think "siege, siege, siege"...
What does a force need to break sieges (attack) or establish an unmovable position from whence to await for support (defend)?
Scouts are an essential part of siege of warfare as is artillery of course. A couple of detachments of scouts (with their excellent coverage of terrain and
ZOC) and 8 Whirlwinds will give the army an IF feel, as well as Paladin and Warlord titans. Now that's quite a few
BL to drop every turn!
Drop the assault marines or keep them as is - a single detachment to quickly plug holes or exploit them. More devastators are key, too, with a nice strong block of tacticals, And don't forget of course about Termies - Titanhammer Termies in particular with
CML.
Keep your speed down - one fast detachment per 1000pt may be appropriate - and a single thunderhawk which the IF as an army have, of course, just don't make as much use of. I mean don't field three and call it an IF force!
No drop pods would also be an idea, and maximise your use of dreadnoughts as well - a lot of players don't like them because they slow troops down but that's not really an issue with your force. One mechanized tactical detachment in rhinos is all you can really field anyway to stay with the fluff as in the attack the objective is to break the enemy line and then exploit the gap rapidly. In the defence it's sit and pound the enemy silly.
Think pounding, siege, and as the scenario requires you're designing either the unmovable object or the unstoppable force of from the old adage. IF are a great army....the Emperor's own banana warriors - and they almost pit the Imperium into a second civil war just as the Heresy came to an end!
Hope some of this helps, anyway
My two cents
S.
GW Skaven 
Mantic Orcs 