The latest version supports any Lemmy 0.18.3 instance. You can see which version a Lemmy instance is at the bottom of site. For example, your instance, reddthat.com, is 0.18.3. Most instances should have upgraded to 0.18.3 already - choose one of those. Any instance running 0.18.3 should work, big or small.
The account you “download” with won’t be touched at all. The account you “upload to” will receive all the subscriptions and blocks of the downloaded account.