On review/discussion sites I find it very useful to know how new an account is, how many posts they've made, and a score (up/down votes) on their previous posts.

I introduced a credibility system for every user, similar to Reddit's karma so people can see how trustworthy the user is in the Buy For Life community. I could also imagine picking reviewers by interviewing them initially - someone who owns and uses the product for a while.

That's not really enough. Any kind of credibility system can be hacked. You should ask two images, mandatory, and one of them should be 'my product', like a some kind of an actual clue, that the reviewer purchased it at least. If you add some basic checking, like a reverse image search on your own db and in the internet to vet out low effort tries, this would be the minimum to prevent hijacking the system.

I had somewhat similar idea for a site like this, but only for negative reviews. This kind of a site stops the incentive of forging fake review, like who wants to forge a bad one, but this still has to be protected from defamation attempts. That's why I was thinking of making the post somewhat complicated, but not impossible to make the system enough hard to protect it low efforts, which is at least 90% of the attempts, and the rest would be up for a moderation team. The credibility system could work only over protection