If possible, I think the karma/rep score should be completely hidden. As long as people can see a number, they're going to try and game it somehow, which incentivizes low quality posts. You can cap total karma, but people will still try to grind up to 5000 and they'll still try to get the highest comment scores they can. That encourages people to make the types of low-effort posts and jokes that often clog up Reddit threads.
The other problem with an overall rep score is that it doesn't truly represent user behavior. If 1/3 of my posts are shitty troll posts, but the other 2/3 are generic low-effort joke posts and memes that people will upvote by default, my rep will stay positive even though I'm a net negative contributor. Likewise, if I make one really popular post that gets 90,000 upvotes, my score will stay positive pretty much forever, even if I troll and harass people nonstop.
So rather than report the sum of a user's post scores, I would propose displaying a "user quality" indicator based on the average score of their recent posts instead. For example, if your average is greater than 5, you'd get a green up arrow, and if your average is less than -5, you'd get a red down arrow, but otherwise you get a neutral icon. You could have other icons for higher and lower scores, but I feel like that might still encourage people to try to game the system, so I'd propose keeping it simple and making it easy enough to get the green icon that you're not incentivized to spend any time on it.
Bertha