Even native speakers might be confused which to use, so the answer is really "both can be correct". "End user" is such a relatively recent term that there is no "standard" way to write it. However as Mick points out in his comment, it can be clearer to hyphenate compounds like this when using them as adjectives.

However, none of your examples make much sense. Every system is designed for "end users" because, by definition, that's who will be using it. The system can be better designed to make it easier for the end user, but saying something is "for end users" makes it sound like the writer doesn't know what "end user" means.

This is a little used car.

This is how I would usually write my sentence for a small car that is used.

This is a little, used car.

Same as above, with emphasis on its size.

This is a used, little car.

While only native speakers would "feel" this, this definitely feels wrong. This post supports this - condition comes after size in adjective order. (See the bullets in the top answer)

This is a little-used car.

This is how I would express a barely used car. I have seen and used this word myself. Note that there is a difference in pronunciation between "little-used" and "little used" - the pause between the words is more pronounced for the latter.

This is a little used-car.

I would never construct this sentence, and I have never seen it. It is very uncommon.

However

With that said, the data shows that I was right a hundred years ago, but currently, nothing I said really holds water. See attached Google Ngram, which shows all three of my points are wrong:

• "little used" was previously very common but its usage is declining
• "little-used" is relatively uncommon compared to "little used" - but "little used" has fallen quite rapidly
• "used-car" was non-existent a hundred years ago, but has overtaken (!) "little-used" *note that this includes usages such as used-car dealer

Ngram Source