Hi there,
There is a default component for Joomla to upgrade to Joomla3 from Joomla2.5. I will use that component to upgrade the Joomla, which will upgrade the database and Joomla core files to Joomla3.
I will then install all extensions for Joomla3 that is used in the website. If there is any custom extension or extension that has no Joomla3 version, I will write the extension for Joomla3.
Finally, I will use a similar template or convert the existing template to Joomla3. I will make settings for site security and install a 3rd party paid extension for security which will cover the administrator login protection, sql injection, bruit force attack and url injection.
I have many experience upgrading Joomla website.
Kind Regards
Mohsin