Archetype work is project-priced, and the main thing that determines cost is how your existing customisations were built rather than what you're asking for.
Name the theme and version. "Impulse 5.4, moderately customised, need custom size-guide section on product pages" gets an accurate quote immediately. Your version is in Online Store → Themes → the theme's action menu.
Separate development from merchandising. Building a promo tile section is development. Deciding what goes in the promo tiles and whether they're earning their space is merchandising and CRO. Both are legitimate hires; they're different people at different rates, and conflating them is how merchants pay developer rates for marketing judgement. If your problem is "the homepage isn't converting," you may want a CRO specialist rather than a theme developer.
Ask how customisations will be structured. The answer that saves you money on every future update: customisations kept in separate section and snippet files rather than edited into core theme files. Archetype releases updates regularly, and how the work is structured today determines whether the next update costs $600 or $3,000.
For Motion specifically, ask about animation performance. Motion's animation and video are its selling point, and they're also the first thing to investigate if the store feels slow on mobile. A developer who knows Motion will have opinions about which animation settings cost the most on lower-end devices.
Check they've worked on your specific theme. Impulse, Motion, Streamline, and Expanse share a house style but differ underneath. Ask for a live store on the same theme.