Hello Minh,
I think your urge to explore is very good.
However, it's always better to research first and then buy. Unfortunately, the reverse is most common. Well, you seem to be a young beginner, and then you really want to own something, even if you get advice to the contrary. But I can completely understand you.
A homework assignment for you now could be to find out about the history and stylistic working methods of the various Nobukuni branches. What are the special characteristics of the (legendary) first two generations (there is still a lot of controversy and mystery here)? What are the special features of the Oei-Nobukuni? When did a change of style to Bizen-Den begin? When did Nobukuni swordsmiths move to Kyushu, especially to Buzen and Chikuzen, and why? Did this have an impact on the way they worked, and if so, what was it? Are there any differences between Buzen and Chikuzen? What possible connections and interactions are there with the immediate neighbors in Bungo-Takada?
When and why did the Buzen Nobukuni disband? Did the remaining representatives possibly move to Chikuzen or Bungo? Why did the Chikuzen Nobukuni survive until the end of the Edo period?
Isn't that a project?
Look at so many Nobukuni blades of various branches with your own eyes. Collect information, photos, Oshigata, and learn to draw Oshigata yourself.
And in 2, 3 or even 10 years, you may be able to provide us with valuable information. You will help others here who are not familiar with the Buzen-Nobukuni to see unknown schools in a different light.
That's how it works, and no other way, if you take a serious look at the subject. Let's go!