Who should i get?

From a gameplay perspective it is not even a question. Strong support is 9/10 times the best investment, and Merlin is top-tier even among them. If all you care about is gameplay value, Merlin is what you want. That said, the game never gets hard enough where you absolutely need him, so most people will advise you to get the one you like the most. As for what type of team to go for, it's honestly up to you. If you end up with Merlin, he's best with busters, so go for buster for the short-term, but just know that Scathach is one of the best quick servants in the game and quick servants in general tend to function pretty independently anyway, so there's no harm in going with that option either. Also, from my experience, people place way more emphasis on team-building than needed. I've literally been using whatever servants I feel like in whatever combination I want for all but the hardest content (ie, challenge quests, certain story fights), and as long as you don't use servants that are weak to enemy classes, you probably won't have much trouble with anything you choose.

Ok this is for real. roll for merlin. Not becouse he is the best (well, he is, but still) simply becouse if you manage to get him, you will be able to use any damage dealer servant in your friend list instead of relaying on your damage dealer and only use support caster (like i have to do )
on the other hand, i hate merlin, so if you like me dont like him, dont roll. ez.
For buster vs quick there is really not many to compare. Whit Fragment of 2030 in game there is no need for any quick oriented servant unless he is complitly broken. Merlin is going to be the last nail in the coffin of the worst tipe of command card in the game.

While it is possible to to get a 5* in one 10x roll or a handful of 1x rolls, it usually takes a LOT of rolls and all your sq and still fail.

EVERYONE will be rolling for merlin and he will probably be in half of your friends list a couple of days into Solomon. Yes it would be nice to have your own, but not necessary, especially when the caster GSSR at new years has a 33% summon rate for waver and Tamamono, who are both excellent supports. I recommend rolling for a damage dealer and just borrow support from friends.

Okita is extremely fun to play with with an insane np gain on quick cards so you can spam that np as much as most art servants. She's rather like jack that way with a bonus of not having a jailbait design.

The old man was fun using in the singularity and it might be fun teaming him with Nitocris and shiki for an instant death squad, but I think I prefer Jack myself.

I hear great things about Musashi, and if you prefer buster she is the saber to go for. Probably won't have the np spamming capability of Okita though with the buster orientation.

***I am going strictly by personal opinion here***

With that said, Merlin adds the most functionality (read: broken) out of the 4 you mentioned, BUT me personally would choose Okita if I only had to pick one for a few reasons.

1) Out of all the SSRs I have I personally use my Quick servants the most save for maybe Jalter as I just enjoy that particular type of gameplay.
2) She is one of the few high tier SSRs I dont have and find myself constantly borrowing as a friend support.
3) King Hassan...OK, but again, I lean quick team anyway so Jack is still better IMO
4) Everyone on your friends list will have Merlin at LV 90, 10/10/10 by the time you log in the morning of his release
5) Musashi is a close second since shes a great buster/crit servant and I will probably roll for her as well at some point.

Again, this is just an opinion as really you dont NEED any of them just to play, but my personal bias leans towards Okita. Putting her, Jack and Scathach on a team together and watching even the highest HP enemy just melt in a few turns is enough of a reason for me.

Roll for waifu.

The real answer to "who should I get" is that you're unlikely to actually get any of them.

133 SQ means 44 rolls. At 1% chance of pulling a 5* per roll, you have a 64% chance of whiffing entirely. At a 0.7% chance of pulling a specific 5* who's on single rate-up, you have a 73% chance of not getting the one 5* you blow all of your SQ on.

So, pick the character that would make you the happiest if you got lucky, go all in and try to get them, and then deal with the disappointment if you don't. Unless you're planning on spending serious money, you don't have the SQ to roll for any character other than the ones that will bring you joy. ;)

As to what kind of team you should build? It depends on what the quest is. Your goal isn't to build one super team that can handle everything; it's to be have enough of a versatile roster that you can put together a decent team to handle the particular fight you're in.

That's why Merlin, Waver and buffers in general are lauded. Being able to use [your buffer] + [the best DPS you can find on your friend list] frees up your options, as compared to having to use [your best (but probably mediocre) DPS] + [buffer from friend list].

"scathach .emiya and mary&anne(rider) which are the only good servant i have"

Then raise a Hans, a Lu Bu, a Euryale and a FSN Cu, if they are already NP5. They have beyond-some-4* performance, trust me.
Also, 10/10/10 Hans Skills and Cu's Protection from Arrows.

As for the ones you will roll...all the ones you mentioned are decent.
Merlim killed the game equilibrium, but that's why your friend list is going to have more samples of him than Wavers and Jalters combined. *I* would not roll for him, also, because Ruler! Martha, Herc, Kiyo and KidGil are my only buster servants...

There will be a new chance to roll Musashi and King Hassan later, I would recomend focusing on saving 4 Okita, in that banner you might as well NP2 your Emiya, also, it get's you CEs for farming the rerun of GudaGuda, where you can get Nobu, another Buster AOE Archer, as wellfare.