Hey there!
I recommend you think in terms of total cost, including future costs. It’s nearly impossible to pick a theme that will meet all current AND future unknown needs. A growing business will usually have to make theme/code adjustments along the way. Even if you find a paid theme that satisfies all of your current needs, you could be setting yourself up for significant expense dealing with future theme updates, tweaks, compatibility issues, etc. Eventually, it may become cleaner to start over with a new theme.
For these reasons, I usually suggest clients consider building on a free Shopify 2.0 theme like Dawn or similar. By going with a Shopify theme, you’ll get Shopify support, easy updates and a stable framework to build on over time. Even if you have to incur a little bit of theme customization expense up front, you will likely save yourself $$$ in the long run. Plenty of great shops run on Dawn (including mine).
There are great paid themes out there too but it’s important to factor in the cost of future updates and support. An example of when a paid theme might be advantageous is when you have a very unique/specific store and there is a paid theme that was designed exactly for that niche. Then the benefits might outweigh the factors I mentioned above.
Good luck to you!