Hi Orly,
There’s not much more I can add to the great feedback you’ve already received including considering the option of a mattress with the option to make layer exchanges (or exchange the mattress itself at low cost) which can greatly reduce the risk of an online purchase.
Post #2 here is one of the links in the tutorial post which talks about how to make a choice that is a good match for your unique needs and preferences when you can’t test a mattress in person. This can be with a more detailed conversation on the phone and Brooklyn Bedding also has a chat service if you prefer it over a phone call.
If you do decide to buy a mattress first (and I would make sure that there are no weak links in the mattress before buying it) and then adding a topper then I would purchase the mattress first and then use your own experience on the mattress along with the topper guidelines in post #2 here and post #8 here to choose the topper (although this wouldn’t be my first choice because there are more uncertainties and variables involved in buying a mattress/topper combination that you haven’t tested in person than there are in buying just a mattress that is already a good match for you).
Phoenix