Hi Alan
Zen Cart is pretty dated. Unless you require specific features I'd recommend OpenCart for an ecommerce solution. It's arguably the easiest to use, and is more modern and clean.
It's also the highest rated here, with PrestaShop a close second:
http://www.softaculous.com/softaculous/apps/ecommerce
If you'd like to see what it looks like, you can can try the demos on that page, or here's the one for OpenCart on their site:
http://www.opencart.com/index.php?route=demonstration/demonstration
Any decent web host will provide a one click installer like Softaculous to have one of these apps installed for you in seconds.
Most decent hosts also let you pay monthly. All you'd need is a starter package to begin with, which you get for as little as £2 per month. If you find after a month it's not for you, that's only £2.
Google often uses the motto "Think big, start small". Keep an idea of what you want to achieve in mind, it's great you already have, and get stuck in and try things to see how they work. You'll quickly find what works and what doesn't, and crucially you'll be building an online presence and history, which can really help your site in the future.
I'd recommend installing a Wordpress blog alongside your shopping cart, for example at yourdomain.co.uk/blog, and using that to write about relevant content to help drive traffic. You can then link to your test products, and share those blog posts on social media and other channels.
I hope that helps.
If you have any questions feel free to get in touch, we'll be happy to help.
Good luck!
Dan