Considering Tristan and Emiya Alter are the two storylocked options, I’d focus on them. Honestly, while Tristan has very nice utility, David can fulfill a lot of his role, particularly the teamwide Evasion. So I’d go with Emiya Alter, if I were you. Euryale is very nice against Males, but lackluster otherwise. Emiya Alter should more than pick up the slack for raw damage.
Ideally, while you eventually want both ST and AOE options, I think it’s more important to cover ST first. Yes, as everyone will tell you, farming is a huge component. But you’ll never “lose” farming nodes, even if you’re not fully efficient. But you can easily lose important boss fights and challenge quests without a good ST damage dealer. So while I love Emiya, I’d go for Emiya Alter first (particularly as he’s story locked and original Emiya isn’t). You can always raise Kid Gil or Tawara Touta for some AOE damage if you need it.