Upgrading 1.5.6 to 3.0.2.0

Hello,
I've been tasked with upgrading a store from an old version 1.5.6 to 3.0.2.0 of open cart.
I've checked out the upgrade instructions, here, http://docs.opencart.com/upgrading/
I've tried taking a backup using the Backup / Restore link, but I get an error 500 on some items when doing the backup. I suspect the server is to busy. Or is there a different way I can backup these items?

I'm looking for advice from people who have done similar, What else would be worthy to look out for?

PHP version on the server is up to 5.4.x

Thanks for your help in advance

If restoring your site after creating a backup generates an error 500, always check the access logs from your host console logs icon or from your root FTP folder of your domain - > access logs folder by verifying the most recent error logs regarding the index 500 with the sentence. Do you notice a recent error 500?

Quick question about backup If I download the entire site via FTP will this also get the SQL folders with the databases in? so it effectively backs up the databases as well?

Download a ZIP backup from your host file manager console. Never download a whole site from FTP. The ZIP feature from the file manager and the download process only takes less than 2 minutes overall. As for the database, no, the backup needs to be done from PHPMyAdmin with the export tab from your Opencart database. Otherwise, if you want them all to be done together, you need to use your site backup tool from your host console.