You being borderline for open fit hearing aids does not matter if you are fitted with BTE devices that are capable of accepting regular ear molds.
You could wear your new aids for a while, and if eventually your hearing deteriorates requiring you to have more power, you could replace the open fit design with a more traditional kind of mold that would allow far more power into your ears.
Modern molds are very comfortable in both open fit and regular fit versions. So if you can get a nice light weight small device that can take both kinds of mold, you should have a solution that will last for many years, even if your hearing does get worse. You can buy a set of new molds typically for $100, and you may not need to do this for a few years.
Just be sure to ask your specialist if your aid can be converted in this way.
Here are some examples of some open fit devices. Note that only the bottom one is capable of being both a traditional BTE and a modern open fit device.
Oh, and to answer your first question, I am okay with Siemens, I like Oticon, but I don’t like Rexton. You forgot to mention Starkey, the largest American brand. Also be careful with price shopping, you do get what you pay for to some extent. Make sure you are comparing like with like, check on the price of after care. If this Audiologist you are going to see is any good, she should be able to let you listen to some real hearing aids right after the test. This will give you a very graphic demonstration of what the hearing aids sound like. Personally, I would never buy from anyone who did not include demonstration of real hearing aids as part of a test.