@Jordan Invision could you get this moved to a more relevant location, as it is nothing to do with the marketplace or the products available from there.
Pretty much, yes - pay money, download package, upload to your site, run the upgrade (if necessary) But there may be other caveats - but never used a nulled version so no idea.
I'm assuming "nulled" means you pirated the software and its been modified in some way by someone to make it easier to pirate or not require a valid license key. Whoever modified the software to allow you to do this may not have the highest ethical standards, and for all you know, your copy may be doing not great things. You should likely treat your data and filesystem as suspect and as if they were compromised.
Rather than copy over valid files on your existing site, I would suggest that the safest way to proceed would be to use the importers to import the data from your existing situation into a legally purchased copy. I would not bring over any themes or plugins from your existing version, and instead ensure that those are acquired separately and from legitimate sources.
Kudos on you for being open and transparent.