Well, you can use zmodeler, but 3ds max is way better, easier, and can save you some time !
Also, regarding the hierarchy i suggest you import a default car model, then replace every part of it with your vehicle. it's easy: you just have to rename the part of your cars: for example "door_dside_f_L0" for the front door on the driver side, etc.
Take some time to study the hierarchy of a default vehicle, see which parts is where, etc.
for the texturing part: use this tutorial: http://www.lcpdfr.com/page/tutorials/index.action/_/vehicle-modification-tutorial/how-to-map-and-make-templates-for-gta-iv-models-r1