Hi,
I have been developing websites with Joomla since 2008, I'm therefore skilled and qualified to handle your project. Lately I have completed a number of upgrades from Joomla 1.5 to Joomla 3.2. I therefore know what it takes. You might not be aware of this but there are no reliable automated tool to upgrade from Joomla 1.5. Therefore the most efficient way is to install a clean joomla 2.5 or 3.2 and transfer the content (categories, users, articles) as well as the extension specific content (VirtueMart in your case) An added benefits is doing away with all legacy content
Also the template engine has completely changed and again you're better off re-designing a new template, based on protostar for instance, the responsive template that ships with Joomla 3.2 and that I can make look and feel like your old site if you want.
I have a question for you, you mention Joomla 2.5 I guess because of Virtuemart. If you don't heavily use the eCommerce features you might want to consider Hikashop on Joomla 3.2 It's a lighter eCommerce extension but it would apparently fit the bill for you. This would save you an additional upgrade down the road.
For upgrades in order to avoid down time I always clone the live site and perform the upgrade on the clone. Once the upgrade is validated the clone becomes the live site. More time consuming but safer.
Do not hesitate to contact me if you have questions.
I look forward to hearing from you and start working on your project.
Regards.
Xav.