What servant should I save for?

I think it would be hard to choose who to save for there are great servants that will be released until December. You may want to focus for at least two servants while you are F2P.
Scathach and Ozymandias for boss killers.
Raikou for optimal farming.
Summer Altria Pendragon for NP spamming.
Nero Bride for offensive support.
Jalter, nuff said.
And Merlin for buster crit meme team.
Still, the number 1 rule is to roll for who you like.

Jack will be your main damage dealer in this event. Pair her with Atalante or Scathach for a 100% quick card effectiveness. For optimal damage, focus on leveling her including her skills. In terms of positioning, I recommend the first three slots in your party to be damager + support + support. Place Mash or any tank in fourth slot, another support in case of plugsuit (Chaldea Combat Uniform) in fifth slot and another damage dealer in the sixth slot.

Depends on your playstyle.

The standard Jalter and Merlin, if you favor buster crit. Brunhild is also great after strengthening. You don't need to worry about buster DPS since Merlin makes them good no matter how bad they are anyway

If you favor art, Tamamo is a must. Art has broader choice, Artoria Archer and Vlad if you love NP spamming on art, Sherlock Holmes, Caster Gilgamesh (he's a 4*, yeah) for art crit, Jeanne for stall team. Katsushika Hokusai is a new face to the art meta on JP and she is the definition of literal monster on any offensive art team you runs.

For quick team, MHX Alter for star control, Osakabehime for durability support, Achilles for AoE nuking