Joomla 2.5 and Virtuemart2 Customisation

Virtuemart 2 customise, category browse page slightly, customise product flypage, Make front page load faster and make some adjustment to checkout. Not too difficult but would like the customisation to be made in a way that future updates won't effect the changes. Looking for competitive prices but also long term business relationship as we have new websites coming every month, mainly in Joomla.