Sorry everyone for the late update but right now I have been a little busy with some work and as of right now my exams are going so I wont be able to publish another chapter for another week, but you can expect me to publish one by December 1 so don't forget to be updated.
Now to answer of your question-
The reason Mangetsu went to the sword shop will be explained in the next chapter and will be something that you will see would give him a major boost in power worthy to be in Naruto's crew.
Someone mentioned about Naruto being godlike. While yes he is going to godlike right now No as of right now he is not. Yes he understands the potential of the fruit he isn't much experienced in using it, which is only possible by fighting strong enemies i.e. grandline. Right now he only as strong as an admiral I would say, but with experience no one will be in competition.
When Ace mentioned that Naruto recently started his journey Whitebeard understood that while he now the potential of the fruit, he isn't battle ready. That's why he said that wants to see the potential of the brat.
And sorry but the crew is already decided.