The Plesk account you already have is shared Linux web hosting. If you want a real domain instead of one of our subdomains, you can just buy a domain from a registrar (we recommend namecheap), set the name servers to ns1.heliohost.org and ns2.heliohost.org, then let us know what domain you bought, we can add it to your account. You'll obviously have to pay the registrar for the domain, but HelioHost doesn't charge anything to change or add a domain.
We do not recommend using .top, .xyz, or any of those other weird TLDs, they tend to have an extremely poor reputation due to their widespread use for malware, spam and phishing. I'd stick to a .com or similar if buying a domain. That said, we can host them if you wish, just be aware that you likely will have issues with email deliverability.
If you want more space, it costs $5 for an additional 1000MB. Since you already have an account with us, $10 will bring your account up to 3000MB. This is a one-time fee, not a subscription. We do not price services in any currency aside from USD, though paypal may be able to do that conversion for you. If you want to buy the space, just send us the required payment, then let us know and we can add the space. The max space per person is 6000MB.
Also, since you already have an account with us, anything you do will need to be added onto the existing account. Users cannot have more than one account per the terms of service.