Customise a Magento 2 Theme

Hi, I'm building a e-commerce website and have selected a ready made theme but it needs a lot of customisation. I will be purchasing lots of plugins for the site and then the theme needs to be customised such that all fields are available. This will be a long term project, I would prefer to keep working with a reliable freelancer or a company as in the future there will be lots of further customisations required.