You need invoicing software. It'll allow them to pay online like a typical product, but the amount and services listed on the invoice vary.
Here's a list of some: https://stripe.com/en-ca/partners/apps-and-extensions/invoicing
Google and any top directories is a great place to start. Here is a list of the top ones business owners place their listings at HERE
For paid traffic you can do Google ads then re-target on Facebook. Create a lookalike audience using Facebook too. Both ad platforms have all the data. You can re-invest for blog content and link building. Nothing like leads from organic rankings. Go to any local events where your potential clients will be at too.
Depends on what kind of website you're going to build.
If you need a simple one-page website, you can use free website builders like Mobirise: https://mobirise.com/
For langer websites, you can consider Wix, Squarespace or other CMS.
Well I would still stick to the freelancers. The competition is higher, so the prices are cheaper in general. You can easily build a little network of freelancers to work for you. Some services are just possible trough white label though, like white label VPN/SAAS and stuff like that. But it will be more expensive. For example, NordVPN has a white label service, but they also decided to do it because the market gets oversaturated. So many companies offer white label because they need to increase their sales or because they're starting to loose money.
This is why it is important to clearly state the scope prior to the project. I am looking at this as a trained, unsuccessful web developer and seeking to become the drop servicer. The drop servicers position is to become the project manager. To become a better drop servicer it is important to learn these management skills that are used to facilitate the project for the client. A key ingredient the drop servicer must also sprinkle on top is marketing the product. Being it is unknown what the product or who the customer is at the beginning a framework I have been looking to is the business model that start up businesses use. Hopefully that makes sense.