Yes, looting everyone else's models makes the deff skulls the funnest army to model
Trukk: a chimera or rhino with the roof cut off and hatches missing (to better represent the trukk's lower armour and open top) is the simplest option. A
DE raider would need a little more work to remove the 'hovering' look, I'd suggest bolting lots of orky bits to the engines to represent semi-successful repairs, and model it scraping along the ground (perhaps literally dragging along bits of junk from the ground) instead of hovering.
Battlewagon: a predator would be a bit too small unless you can extend the hull somehow. A land raider might be a more suitable conversion base?
Deff Dredd: yes, sentinel chainsaws should look fine as replacement dredd 'hands' on the end of the 'arms'.
Biker Nobz: also consider chaos bikes and scout bikes? Regardless of what mount you choose, prepare for some serious leg surgery to make the ork riders sit properly.
The only problem with converting sentinels to kans (other than shortening the legs) is that it's rather expensive. Have you seen the grot tanks at
http://www.forgeworld.co.uk/Warhammer-40000/Orks/GROT-TANKS.html ? I'm doing simiilar conversions to count-as kanz in my army - it's a great way to use up the bits box.