Performance and speed server:
Some VPS prices you have seen for dedicated versions and they are quite high due to the fact that you have the whole server to yourself. You don't need one at first. I think that for you and your requirements, a VPS like OpenVZ (starting at $4.5) or a slightly better KVM VPS will suffice in the beginning. For example, ramnode.com from your market :2GB SVZS 2048MB 2 Cores 60GB 2TB $7/mo. That's the cost of one coffee at your place. Such a server will calmly support you MySQL database and ABMaterial server up to 1000 users.
I one simple ABMaterial site for data entry and uploading continued to go on a VPS server with 256MB RAM for $7/year.
It is important in my opinion to choose servers with MVMe disk. They are fast so that you can safely the first database you can use SQLite.
In addition, you can use free services such as cloudflare, you plug in your domain, make a redirect to your own VPS. The plus side of cloudflare is that it protects your server, because you can not see the IP of the real server, so there are fewer attacks on your machine, and it gives a proxy for your site to the extent that it can save up to 90% of the transfer from the server to the client.
I have the site on a smaller server and with cloudflare works as if it were a dedicated server. I think for you in the beginning it is enough. And as you develop the application in the future you can calmly move to a more powerful machine.
So far I have only followed the template that comes with the examples, but can the feel and look of ABMaterial template be customized, for example can the top navigation/title bar be customized, removed or hidden? As I mentioned I'm not an expert with JS, but If I decide to go with ABMaterial I would like to be able to have full control over customization as well.
Yes you can hide, modify the bar, menu, colors, position. You don't need knowledge of JS or CSS for most of it at first. As I wrote earlier some things you will do quickly , others you will need to add code, and still others you will probably need to build your CustomComponent or insert your own JS into the code. This I can't give you an estimate because I don't know what you want to do. For example, you want to make PayPal payment so popular with you. I don't think there's a good B4X library for that, but they give you their own API and you can write the service yourself, or what's faster and proven use e.g. Python which has great tools ready for that and link to your service.
Overall, I think you will need a VPS anyway, whether you want to use ABMaterial or another Banano framework. However, something must be doing to you the logic of your service. Without VPS, I guess the only option is php and using web hosting services.
When choosing banana, you need to focus on learning frontend, sometimes js, css and html. Then writing an API server with website logic support. You have to connect everything to make it efficient and safe. In B4X there is something about API, you can write your own API yourself. I prefer to use e.g. FastAPI in python. Well, here comes another programming language.
However, for you, ABMaterial will probably be ok at the beginning because you don't need to know js, css, etc. to create a nice website. You don't need an API because both frontend and backend are one code in one server. In my opinion, debugging ABMaterial is much better than first debugging the Banano framework frontend and then debugging the API server and testing API security with the website.
The big advantage of the Banano framework is its powerful front-end website building tools. You can do miracles, but if you don't know the tools, it will take a lot longer to create the website. Hence the market division into frontend and backend, ABMaterial has everything in one.