[snowwrestler]

I think you are smart to be nervous. There is risk when you don't know very much but are getting paid by a company to do technical things to enable their business.

Consider what is commercially available. Selecting, configuring, integrating, and supporting software services is still something you can charge for. Small companies pay for efficient results, they don't typically care how much code you personally write.

You could build their marketing site on a fully hosted solution like Squarespace, Wix, Weblow, etc. Or if you want to use Wordpress, stick to something like Wordpress.com or WP Engine with a commercial template and all auto-updating turned on. Make it easy on yourself and outsource security to folks who know what they are doing.

Remote desktop and invoicing also sound like needs that can be met more efficiently and securely by available commercial solutions.