Sending platform (ESP)
This is the one place it's worth paying attention to reputation, not just price. MailerLite and Mailchimp both have usable free tiers for small lists and cover the essentials — automation, segmentation, basic reporting. If a client is ecommerce-specific, Klaviyo is the industry standard once the budget allows it, because it ties directly into store data in a way general ESPs don't.
Design
Most ESPs now have a drag-and-drop builder that's good enough for 90% of campaigns — resist the urge to overcomplicate this early on. When you do need custom graphics, Canva covers social-style assets and simple email headers without needing design software.
Automation beyond email
Zapier (or the free-tier alternatives) is worth learning early, because a huge amount of freelance email work involves connecting a signup form, a CRM, and an ESP that don't talk to each other natively. Being the person who can wire that up, not just write the copy, makes you more valuable to a client.
Reporting clients actually understand
Your ESP's dashboard is built for you, not for your client. A simple monthly one-pager — open rate, click rate, revenue if it's ecommerce, and one sentence on what you're changing next — builds more trust than forwarding a screenshot of a complicated dashboard. Google Sheets or even a clean Canva one-pager does this job fine.
Where to actually spend money, once you have clients
Pay for the ESP tier that removes real limits (list size, automation count) before you pay for anything else. A paid ESP plan you're using properly earns its cost back fast. A collection of unused "productivity" tools does not.
The real lesson
Clients aren't hiring you for your tool stack. They're hiring you because you can think in terms of lifecycle, write copy that converts, and report results clearly. The tools just need to get out of the way of that.