Is there a texture editor where you can take what you did on the one side of the model and put it on the other side of the model? 

Photoshop, Paint .NET, GIMP, etc. Copy and paste the design, then flip it so it's the same. (You will need to copy and paste the text separately.)

I'm almost positive you can do that with any decent image editing software (Photoshop and GIMP, to name a few).

Do you have to ask the author of the model if you want to publish a texture? I assume so.

Do you have to ask the author of the model if you want to publish a texture? I assume so.

I'm pretty sure you don't

Do you have to ask the author of the model if you want to publish a texture? I assume so.

 

No, only if you are gonna release the car model with the texture...it's best to just give credit the to the car author if you are just releasing the texture.

 

Do you have to ask the author of the model if you want to publish a texture? I assume so.

for skins/textures no but for using models yes. and you can easily do that in paint.net. make a layer and name it skins and make sure theres no logos or text on it. then make a layer above that and name it text/logo or whatever you want. select the skin layer and copy your skin from one of the sides. then press control + N and press enter. now press V, Layer in the top menu, flick horizantally and then copy your new flipped image and paste it onto your skin layer over your template.