We're using eShox 3.9 and it works okay after tons and tons of hacking.
eShox 4 is in beta an supposedly will be released "real soon now", but I wouldn't hold my breath. It is a complete rewrite from scratch and looks pretty good, but the support has sucked big-time as the developer has been off coding in code-land. Hopefully that will change once it is released. Other problems are no comments in the code whatsoever, so if you want to hack the PHP yourself, then either you need to be good, patient, or both. Another drawback is that the community is small and I mean really small. I'm a PHP-novice and end up helping out as much as anyone.
I'm evaluating eShox4, Zen-Cart and webassist for my next project, but haven't made any conclusions yet. osC, while it has many great contributions, is a morass of code AFAICT.
Zen Cart is a bit better and the main devs seem more involved with the community. They are rolling out an incremental release of their next major version. I'm not sure I agree with the strategy, but the current version is a snap to install and is easy to evaluate.
Webassist I've just started looking at and have no conclusions. If anyone out there has used it, I'd like to hear some first-hand info if possible.
Looking at your requirements, I might be inclined towards eShox 4. One of the major new features is product management, i.e. getting your products in and out of eShox. If you are selling car parts, then you will likely have tens of thousands of items. If those are in a DB and eShox4 supports it, then it might save you tons of time. Another thing is speed. I have seen many a thread at osC where the thing just collapses under load with large product catalogs. Since eShox4 is written from scratch and the guy seems to be a proficient coder, it might scale better.....
There's no silver bullet in my experience.