You're headed in the right direction with your fixes.
Usually this indicates a pretty inefficient code setup. Wordpress is all basic (old, gross) PHP and when someone write an import plugin it's just going to start at 1 and endlessly work towards the last vehicle. You're likely hitting the Maximum Script Execution time, rather than the file limits since they're not actual uploads.
If it started processing properly you didn't hit the max POST size limit.
If the files are being downloaded from URLs you didn't git the max UPLOAD limit.
That typically only leaves an actual error (ie: broken image link) or a max EXECUTION TIME limit in PHP being hit.
Usually this limit is set to ~30 seconds so that's very possible.
I would recommend changing that, but changing back when you're done.
The better solution (if supported) is to break the file into 40 vehicle chunks and import piece at a time - bit of a pain for 1000 vehicles.
Alternatively, you can run the import on a separate or local installation and import the images and database manually.
Alternatively, PM me and I'll help 
