but his stats as 1 star make him unusable.
I wouldn't say that. There are vids of people clearing high level content with even un-Grailed *1, so it all comes down to strategy and RNG, really.
As for the Grailing, among the 4 *3 you mentioned I'd only level 80 Medea and Euryale because of their usefulness. If NA is already in Camelot, then you'll soon realize how powerful Euryale is.
Also, what I forgot to mention was I Grailed other Servants besides Umu because of their services to me + gameplay usefulness. Again, the choice to Grail is in your hands. Whether you want to show your love or gameplay comes first, it's all up to you. What I can add is make sure you use the Grailed Servants a lot, otherwise you'd waste Grails for basically trophy waifu.
PS: If you guys play on NA can you add me? If you friend list isn't already full of course. XD
I don't play anymore, sadly.