Mummu probably since you can ride that full set and get a decent amount of accuracy if it's +2. Ordinarily, I wouldn't recommend anyone using a full set of Ambuscade gears, but if it's a new player who needs something to get him into higher tiers of Ambuscade or other Content (without buying gear from mercs), this is my exception. You will get a lot of use out of the Hizamaru Legs +2, so I'd definitely +2 that one first. Daken for Ninja is important, so having a combination of acc, racc, and agi is very good, hence Mummu. Macc also helps you out a for landing debuffs.
Accessories: Please just visit the ninja guide. They have this broken down into entry level sets (everything you asked is there, too!)
https://www.ffxiah.com/forum/topic/33470/the-sealed-dagger-a-ninja-guide/
But to answer even further
Oboro Katana is good, but it's not quite there yet in terms of a great one. You should aim for it since, again, its free with only time being your rival. It gives a very good (again) acc, macc, and racc boost, and even totes your MAB as well. Don't think you can DW those yet since they are rare/ex, but it's a good offhand for now.
You can also get by with
Raicho/+1 as a main hand Katana, which is also relatively free if you spam Vidmapire. You can get a group for him. I spammed a few dozen Vidmapires a few days ago and saw this pop up frequently, and he's not a hard NM if you have a good group or strong friend helping.
Tancho +1 is decent, and you can probably solo him just fine if you are desperate for one quickly.
Of course, you can skip all of that and just buy a Kanaria from a merc (if you so choose) and you'll be set with your main hand, once you augment it accordingly.
I'd recommend getting a
Shigi and augmenting it if possible. It's a very good accuracy option. The amount of money it will cost you, though, is comparable to just buying Kanaria from a merc, but still a good acc swap to have when you need it. Since it will cost you gil, I would prioritize this far below Eletta Katana + Kanaria, and only get it if you really want that hefty acc bonus, as it is a good one-size-fits-all Katana offhand.
There's also some stuff like
Mijin which is a decent offhand if you do Path B.
There's a decent handful of other Katanas as well, but the trouble to acquire them may or may not be worth it, given the'r stats and skill, or potential augments. Research the below if you feel like:
Izuna,
Aizushintogo,
Achiuchikapu,
Jushimatsu,
Taikogane