Advice for Whale

I'm inclined to say Riders as their high star weight makes them capable buster crit dps. Saber pool is pretty unimpressive from a gameplay perspective.

Gilgamesh would be a good target. Benefits a lot from the buster buff and has an increased star gather rate to capitalize on the increased stars as well as increased crit damage.

If you don't want a rider, you could hope for Altera in the saber banner or Karna from the lancer banner. Both have buster and crit skills as well as a buster focused deck that pairs well with Merlin.

I think Altera in particular is quite underrated. Her second and third skill make her a pretty good buster crit servant actually. She can create and absorb her own stars in one turn for short critical burst. Plus she has amazing sustained damage thanks to her high stats and her buster buff, which is active for 3 out of 5 turns.

For GSSR Rider is the best with Merlin for damage dealers. Quetz and Ozy do the best single target damage in the game. If you don't want to roll Rider honestly the other banners are pretty mediocre as far as challenge quest performance goes for being paired with Merlin and 2030.

The banners to definitely stay away from as far as gameplay goes is Saber and Archer. Reason being their lack of damage, low star absorption, and low hit counts for Saber. For Archer Orion is a very niche servant performing very well in arts teams against male saber bosses, but skills have little impact outside of that. Arjuna has a really bad deck type and NP when compared to other AoE archers, Mainly Gilg and Ishtar. QAAAB with buster np does not allow him to np brave chain and his instant kill mechanic is not reliable unless you are fighting waves of Divine which isn't as common. Tesla on the other hand is a god once his Strengthening comes out. Which isn't until next anniversary of 2019, but you will be rolling into a 33% chance to get him which isn't ideal.

Banners to consider outside of Rider would be Assassin Extra. Jack can generate a large amount of stars and help to increase the amount of stars you provide your damage dealer with one copy of 2030. She also destroys female riders which are very common in many quests and challenges. Jeanne Ruler fits into stall teams and after her np strengthening she becomes an unkillable unless you are fighting enemies with pierce invlun which isn't very common.
Caster has two of the strongest supports in the game. Tamamo and Waver. Tamamo is the cornerstone of arts teams, and Waver fits into literally any teamcomp you can think of. Xuanzang Sanzang is a very unique servant in the fact that she is a strong caster damage dealer. She can pair with Merlin to destroy any assassin in the game, but outside of this she still suffers the .8 damage modifier of the caster class.
Berserker has one thing going for it and thats Cu Alter. He is a 33% chance, but he is able to solo very high end content due to his incredible defensive skills. The other servants are very bad. Vlad III is an arts berserker who doesn't generate np very fast. Nightingale is Merlin but weaker and takes double damage from everything.

As far as your other question about Musashi she is the strongest Buster Saber in the game. With Merlin and with 2030 she will be able to steam roll through just about everything you throw at her.

Other banners to look out for in the future would be the King Hassan banner as he is the strongest damage dealing assassin and he is a buster based which works well with Merlin. Gilgamesh Archer who will be on rate up during one day for New Years. (Separate from the GSSR) Ishtar's second rate up in May of 2019, Raikou's rate up in June during Onigashima rerun.

If you would like to look at all the rateups for the next 2 years, I created a spreadsheet. You can find it here.
https://docs.google.com/spreadsheets/d/1bbBeZT3opglp7XTZmGs5jM3nkZmbDu7IqwYcQjWj0W0/edit?usp=sharing

I hope this helps.