Sucuri Firewall or MalCare – website firewall and malware protection.

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The premium version gives you access to all the features you need for lead generation, including PayPal, Stripe and Square payments, conversational forms , email marketing integration and more.
SeedProd Pro – Adds a drag and drop WordPress page builder .
WP Mail SMTP – Improves email deliverability and fixes WordPress not sending emails issue.
Uncanny Automator – WordPress automation plugin that helps you connect your website with popular tools and services without any code.
Marketing

Constant Contact – One of the best email marketing services , list of cambodia cell phone number especially for small businesses.
OptinMonster – Helps you convert website visitors into leads and customers. You need it for lead generation and conversion optimization.
MonsterInsights Pro – The best Google Analytics plugin helps you see where your users are coming from and what they are doing on your website. This allows you to make data-driven decisions to grow your business.
All in One SEO Pro – Improve your website’s SEO ranking.
Security

Duplicator Pro – The premium version of the plugin offers automatic backup scheduling, remote locations for your backups, drag and drop site migrations, and much more.
There are many more plugins and tools you may want to use. We recommend you try the free version first and see if it's right for you. Many premium tools are available in free trial. Use this to see if you really need the tool.

As a business website, you will naturally want to spend money to grow your business. We don't discourage you from spending money if it makes sense and you can afford it.