Hire a designer to build a landing or use a template?

Template first! Unless you have a lot of cash to spend.

The first attempt to something that converts 1000% better will be totally different. Learn with minimal money involved until you get at a descent level.

Ask people here for advice :)

I have some free templates with Bulma.io they are responsive 100%.
They are using the Bulma's CDN si is just a template to jumpstart your project.

https://github.com/michael-andreuzza/Free-Bulma-Templates

Use this as a template if you need forms on your website;
https://www.wufoo.com/

Reaaallly really easy to use and very customiz-able.

I always use a designer. The way I look at it is that it's hard to validate your idea if your landing page isn't top notch. I can normally get out the door for under $400 with the designers I use and they are killer (college students). I only make a landing page 3 or 4 times a year though because I do a bunch of research before building it to make sure it's something I want to ride out and the addressable market looks large enough.