No, there isn't a limit to the number of custom fields you can create, and I don't believe 18-20 is going to cause you performance issues.
When it comes to plugins, if it's not on the
recommended list of plugins, it's up to the individual whether they want to test it out. In that instance they wanted to be able to search multiple fields at once (i.e. price and make). At present you can use the search field to look for a specific keyword.
The upcoming release of
ClassiPress (v3.1) which is due out shortly, is going to see further improvements to the search function, but specifics of this have not yet been released outside of what is seen on the blog.