Joomla! migration 1.5.x -> 2.5.x

Since official support for Joomla 1.5.x ran out and our provider changed its server's to PHP 5.3, we are looking for someone with experience in migrating Joomla! from version 1.5.25 to newest 2.5.x. The website's address is www.scniederbipp.ch. The content language is german; however, admin login (backend) is available in english. Additionally, the following components need to be migrated too: - Phoca Gallery (currently installed: version 2.5.0); this point is particularly important, as there is a huge amount of pictures already uploaded to the gallery. - Poca Guestbook (currently installed: version 1.3.2 Beta) as well as the following module: - mod_TwitterWidget (currently installed: version 1.0.0); however, this module can also get replaced by another suitable one The template currently installed is JA_Purity (V1.2.0 12/26/07, [login to view URL]) which is not compatible with Joomla! 2.5.x; however, we are going to change the template anyway, so this point shouldn't bother you. We will most probably use one of the latest [login to view URL] templates. The website is running on a Hostpoint server ([login to view URL]): PHP version 5.3.10, MySQL version 5.1.58. Let me know if you need more information! Best, Pascal