Thx. For looted wagons, do I need to build it by myself or there are some official models?
Build it yourself, that's sort of the entire point of 'Looted'
Grab something that at least looks vaguely vehicle-ish from another faction and let your inner Mek loose on it until you feel it's Orky enough... For obvious reasons, it's easiest if you're handy with plasticard and/or have a sizable bits bin. But a knife, some green stuff and a lot of creativity will get you far...
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.