Hello,
I am a Joomla developer and interested to upgrade your website.
Generally 2.5 to Joomla3 is easy only the things is templates and extensions. If the extensions has it Joomla3 version than no problem, if not then we have to use alternative extension. For the template , we can buy the same template for Joomla3, if not available then we have to build the template.
I have experience upgrading Joomla site and so far upgrade and migrated more than 50 Joomla site. You can see my work history here, most of my jobs are with Joomla.
Thank you.
Mohsin