Need ST Assassin, F2P, when to roll?

Personally, I would save the SQ for a later banner. Only 60 SQ is a really low chance at rolling a 5 star servant. If you like the servants for the Valentine's rate up as much or more than Gramps, then you should probably save. There's always a chance you'll get lucky though, so really it's up to you whether you roll or not.

As for ST 3 star saber, Caesar is probably better to raise. He does support and damage pretty well, and you're likely to eventually get him to NP5 if you keep rolling the FP gacha. Bedivere is unfortunately story locked, so you can only roll him on the story gacha or banners that specifically have him on rate up.

I'd raise Caesar, he's got more utility, though teaming Seigfried and ST George ain't a bad idea.

as for your main question, if you love gramps I'd recommend rolling while you've got the chance, he's more damage but less utility then Jack, but if you don't mind who you get out of best daughter and MXA I'd save and dump then.

Or you could roll everything youve got now and hope hope gramps then if you don't get him save the solomon quartz and tickets for the jack/mxa banner.

In about a year, the Garden of Sinners event rerun should feature Ryougi Shiki as a welfare assassin. I've only ever used her from a friend support, but she's pretty good, and ranked highly.