You'll find a lot of recommendations in various threads in the death metal section:
My favorite newer bands with old school elements are Disma, Funebrarum, Cruciamentum, Dead Congregation, and Grave Miasma. But there's so much good stuff out there that I get kind of burned out going through it all. Have a look through those threads first, and let us know if anything catches your ear.
At the very least you need a good-quality recording deck in good working order, which can be a problem these days since most of the high-end consumer gear is decades old. Lots of little moving parts equals lots of different ways for things to fail. I haven't dealt with any of the currently available new consumer-level tape decks to judge their quality, but I wouldn't waste money on a knockoff brand. Keep in mind that even if you find a good deck, you'll still be making these one at a time, unless you invest in a real duplication machine that can run a few copies at once.
Then there's the cassette tape itself - the actual tape inside the shell. You can look around on forums for better/more information than I can give you, but it seems to me like high quality tapes are getting harder and harder to find. Normal-bias cassette tape is ok, but brightness/clarity/overall fidelity compared to the source material (in my experience) is inconsistent and somewhat lacking at best.
One of my bands is actually dealing with this very issue right now and it's been a shitshow. Our first duplicator did a terrible job and had no quality control; the tapes were unusable and the company wound up refunding us. The second company did much better overall, but doing a quick spot check this morning, two tapes out of the six that I had time to check were faulty. And this, I stress, is coming from a "real" company with "real" equipment. The kicker for us is that our release show is tomorrow night.
All of this is on top of the other pitfalls that tapes are subject to, especially fragility and loss of fidelity over time. In my opinion tapes are a waste. They can definitely sound pretty good, if the stars align in your favor, and it's neat being able to record and listen with that kind of spontaneity, but no amount of nostalgia or glorification of physical media makes it worth the hassle for me.
Seven-strings, along with various other numbers of strings on various related instruments, have been made for hundreds of years. European classical music has been written for seven-string guitar since at least the early 1800s. Wikipedia has the first known electric seven-string being made in the 1930s, and solid-body models were developed during the 80s.
NSBM isn't a distinct musical style. Usually it refers to bands with racist or nationalist lyrics, but sometimes bands are labeled "NSBM" based on the beliefs or political statements/affiliations of their members.