Is this ethical: "I like your paper and if you submit it to Journal X, I will be the reviewer"
If we replace the word "reviewer" with "editor", this would typically be seen as normal behaviour. I.e., editors often have pre-submission discussions with authors about the suitability of a manuscript for their journal.
In the case of editors, it is a standard part of their role to have discussions with authors about submissions and in some cases encourage authors to submit their work to their journal.

In addition to "editor in chief", many journals have associate editors and the like who act as editors for submissions related to particular topics. They also often have some scope, although typically less than the editor in chief, to encourage submissions.

Finally, the person you mention sounds like they might be a very active reviewer for a journal and quite often get asked to review submissions on a topic related to your paper. 

I think the ethical issues relate mostly to whether everyone is acting honestly and in good faith, and that conflicts of interest are not perceived.

Editors are allowed to solicit submissions, and they almost always know the identity of the submitting authors. They are meant to be well-regarded members of the academic community and are trusted to make publication decisions based on academic merits and not based on friendship or other bases.

Reviewers are a little different. Reviewers are meant to review in good faith and base their decision based on academic merits. For journals that expect double blind review, I think that encouraging authors to submit to a journal where they are likely to be a reviewer is problematic. For journals that only have single blind review, the issue is more one of honesty. Specifically, the reviewer should generally inform the editor if they encouraged the authors to submit the paper. That way, the editor can integrate that information into the assessment of that reviewer's review and assess it for bias.

More generally, the implications of the reviewers comments are problematic. A more appropriate comment would be "I think your paper is good and it would be a good fit for journal X." or "I know that the editor of journal X would be interested in your paper." By focusing on the idea that they might be a reviewer, it implies that they might give a more favourable review than a random reviewer, which from some perspectives implies a biased review process.

If you are submitting the paper to that journal, you could remove most ethical issues by acknowledging in your cover letter that your colleague and potential reviewer recommended that you submit the manuscript to this journal. This would allow the editor to consider whether they want to let your colleague be a reviewer, and if so, how their review should be treated.