Ah, I see.
For a subscription it’s a bit different. You still need to find a WooCommerce theme that you like on the market, but there’s different plugins involved for subscriptions.
These two plugins work together perfectly (same authors) to create subscriptions on a WooCommerce theme and restrict pages or content to only users with active subscriptions:
The custom form is going to be either easy or expensive to do, depending on the options it will contain.
WooCommerce has a feature called “variable products” (which can be turned into subscriptions using the plugin above) that will present your users with options on the product (think “shirt size” or “color” for example). Selecting different options can result in different pricing, effectively letting them customize the subscription/product to their liking. This is built-in functionality and requires no extra plugins.
If you need something more advanced than that, you’re looking at custom development which will raise your costs a bit. You can try and find a developer that Envato has vetted at Envato Studio who is willing to customize WooCommerce as necessary for you.
Once you install your theme on WordPress, you can then simply upload the plugin files and activate them, and everything will come together. The themes and plugins might have some setup steps for you to follow, but they will include detailed instructions in their downloads.
P.S. Both plugins above, and all themes on the market, have live demos you can try.