Hey Bogdan!
Happy to answer your questions! 
The “Pro” plan was actually the original plan I started with when I launched Magic Pages. It included everything – basically, the full package. But as things grew, I had a lot of people reaching out who wanted a more affordable option with fewer features.
So, the Starter plan was born out of that demand. The limitation on custom themes isn’t due to any technical constraints, it’s purely a business decision.
The Starter plans are just about covering their own costs. They leave me with around $1.50 per month per plan (one of the reasons for the price increase I mentioned in my last post above). They’re great for people starting out (hence the name), but I can’t really run a sustainable business on them alone.
The idea is that if you’re serious about your project and want to customize your site more, the Pro plan would be the way to go. I still think it offers incredible value when you consider the SLA, the email usage, and the fact that everything is managed for you.
When you’re testing the market, do you really need a custom theme? If you do, I’d like to think that $12/$15 per month (old/new pricing) is a fair deal for that level of customization and service. Otherwise, there are other Ghost hosting services as well that have no limitations on custom themes, but are a little less managed (e.g. no managed email, etc.). Maybe that could be a good option for testing the waters?
For that, you can use the 14-day trial. It’s only limited in the number of emails you can send, but it basically represents the Pro plan. After the 14 days, you can decide whether Magic Pages works for you and whether the Starter or Pro plan suits your needs better.
Great question! Some differences are that Magic Pages offers custom configuration options and the ability to download full backups from the customer portal, which you can’t do on Ghost(Pro). Also, subdirectory installations are possible without any add-ons on all plans.
Hope this clarifies things! If you have any more questions, feel free to ask. Always happy to chat!