The single most important thing on these themes is establishing how far behind you are, because that determines whether you're buying an update or a rebuild.
Find your version before you brief anyone. Online Store → Themes → the theme's action menu. Turbo v7.0.0 or later, Flex v3.0.0 or later, Retina v6.0.0 or later, Responsive v8.0.0 or later, Parallax v5.0.0 or later, and Superstore v4.0.0 or later are supported by Theme Updater. Older than that and you're likely looking at a migration rather than an update, which is a different scope and a different price.
Ask for the update-or-rebuild call up front, and expect an honest answer. If your customisations are light and you're a version or two behind, an update is straightforward at $600-$2,500. If you're on a vintage version with years of accumulated edits, rebuilding on a current version sometimes costs less than untangling the old one — and leaves you better placed for the next release. Ask any developer to price both paths and explain which they'd choose and why.
For Turbo, ask about the preloading behaviour specifically. Turbo's speed advantage comes partly from page preloading, and it interacts with apps and tracking scripts in ways that aren't always obvious. A developer who knows Turbo will have specific opinions about which apps degrade it.
Ask how customisations will be structured. Separate section and snippet files rather than edits to core theme files. On themes with this much history, that decision has usually already been made badly by someone — getting it right now prevents repeating the problem.
Ask for a live store on the same theme and a similar version. Turbo and Flex differ, and a five-year-old Turbo build differs from a current one.