Cost will definitely depends on your specific requirements, and the web agency.

For example, for the mobile repair side, are customers able to schedule a date/time for the repair and have to enter in their location? Does there need to be a system in place to ensure there's no double bookings?

That's just the tip of the iceberg, but a good web agency will dig in deep and ask you a lot of questions in a discovery session to determine your requirements, so that they can propose a solution that'll meet your needs.

The first step is find a web agency who has experience building similar websites. Then, have a conversation with them. Look for professionalism, clear communication and honesty. Ask if they have a minimum project budget. If they check all those boxes, consider paying them to do a discovery session to help you determine all of your requirements (paying for 2-5 hours should be enough). This is a useful exercise for you as well, so that you are clear about what you really need (even if you don't end up using this agency).

After that, the agency could put together a proposal for you.

Generally speaking, a website of this nature will not be less than $3K to build. It could easily be upwards of $20K, depending on all your needs.

Your best bet is to keep things as simple as possible, and launch with the essentials rather than building everything at once.

Use tools like Squarespace, LemonStand or Shopify to get a lot of features out of the box and reduce development cost. After launching, gather qualitative feedback from customers, and measure traffic patterns and conversion rates using Google Analytics. This data can tell you what to change, add or remove from your website. That way, you're building what customers actually want without just making guesses.

Best of luck on your project!