Whether you're confident in your coding knowledge/ability, or if you're paying a developer to make the changes, you'll probably want the theme files first so you know what you're dealing with in terms of figuring out what is involved with the customizations. However, in saying this, if you were to hire a developer from the
AppThemes Certified Partners page, then you could request a quote for the work first, as these developers will have already worked with ClassiPress enough to have an idea of what would be involved. You'd of course need to purchase the theme before any work goes ahead though.