Ugh damn forum trolls.
Ok you can have different body parts, but unfortunately you have to change car model files to do it, and someone has to have made the model already, unless you'd really good at 3d modelling
Upgrades - There are programs about to modify the power of the cars, but you can only use it online if the host has the EXACT SAME SETTINGS.
Go to www.lfs-database.com
and search for LFS tweak Z (it will NOT work with test patches, only LFS version Z stable)
Hope this helps.