How would you start her out?

don't, make her just an adversary that annoys the crap out of the party with her subterfuge.

What if it was just a hand, and not mara specifically? Or do you think that a hand should just be left as a story point?

There are some stats for an Emperor's Hand NPC in the EotE core book, page 402. Those give you a pretty good starting point, and then you can tweak things to suit the role you want them to have in the campaign and to accomodate the PCs' power level.

Thanks I must of missed that.

I'm inclined to agree, you can use the "base" Hand stats, with some extra, or just two, or three, things, and use her more like the did Vader, and Lando. As much as I love having famous NPC stats, this system can really draw from the whole "you only need the block if they actually find her", with her being several levels of intrigue separated from what the players are actually doing/encountering. I don't want to spoil anything for Mass Effect: Andromeda, but there is a "mysterious benefactor" we are somewhat introduced to. I'll tell you we never concretely learn who it is, in Andromeda 1, but when we see stuff they were in, they do this thing with constantly shifting faces, of numerous different aliens, and, I think, even occasionally changing fake voices. Even if you figured out what they were now trying, and stopped it, you might never actually figure out who the real person, behind the charade, actually was, and a Hand, especially Mara, could do that, too.

I say this because, to me, this system can be weird with characters. You get writers who can build Yoda at 2,500 XP, with only FR 4, and it seems to fit neatly. Many Inquisitors seem to make do with two talents, and one Force power, if they have enough minions. Said Hand wouldn't, necessarily, need anything much beefier than the base Hand block, by that logic, and adding too much more is just being cheap (me). Depending on the fluff you use, she's got a certain portion of the Emperor's own powers, since he rarely needs to use them, and can reclaim them whenever its convenient, so she might have some of his future-sight, and bail before they even know they've got her. Stuff, stuff, stuff. Sorry if this isn't particularly helpful, and i wish you the best of luck.

If I were to make on as a PC, I'd probably mix Infiltrator with Force-sensitive Emergent (or possibly Exile). Though capable combatants, Hands seem to have been orders of magnitude more subtle than Inquisitors. As a statted NPC, an Inquisitor build focused on stealth, Streetwise, Skulduggery, and social skills, with a few duplicitous talents and a Force power (or two) would do just fine, I think.