For this task, you really need a native English-speaker. However, only a limited number of English-speakers really know how to use proper grammar. For example, how often have you seen someone properly use a semicolon? Here is an example:
Our products provide superior services; our competitors just can't keep up!
Proper placement of commas, apostrophes, and other punctuation are also used erroneously by many English-speakers. My writing here as just a sample of what proper punctuation should look like. If I were to proof-read your posting, here is how I would have revised your description:
We are looking for a proof-reader for a website full of content. The website is organized into 3 sections: Products, Services, & Sectors.
Microsoft Word indicates that the total content is ~7500 words.
We need someone to proof-read the content and improve the grammar; we do not need a spell-checker.
The "sectors" category of our website largely contains information specific to certain industries, including: Telecom, oil/gas, manufacturing, logistics, facility management, food/beverages, etc.
In this section, we want to explain how our products and services can benefit and serve people in these fields. You should be able to read the descriptions about these services, proof-read them, and improve the content in order to add value
so you should read properly the products and services sections, and proofread it, and improve the content.