General help required with Magento 1.7.0.2. Making 'code' changes and fixing minor bu

We're currently running an online store using Magento 1.7.0.2, however there's a few changes we'd like making which are not directly available as part of the software.
We have an issue with postage pricing and the gallery which also need resolving.
We also have quite a few changes that want making with many more long-term projects so the winning bidder will likely be kept on indefinitely.
The store is vital to our business so downtime must be kept limited and bidders must be very familiar with Magento. The winning bidder must also be available to speak to by phone I don't have time to spend all day on Skype / MSN etc.
A list of jobs and more details can be provided if required.