Hi there. I'm surprised you are starting at 30mg as the usual starting dose is 15mg and mirtazapine is another oddball that is more sedating at lower doses.
Yes, it will put weight on you if you don't watch your diet; but it won't continue to put weight on after you come off of it.
Withdrawal, IME, isn't as awful as many of other ADs. If you are concerned, or have had rough WDs from ADs in the past, ask your pdoc to switch you to fluoxetine (Prozac) and taper off of that. Prozac has an extremely long half life, and tapering off of it is pretty painless compared to most of the other ADs.
RXing an SNRI/SSRI for sleep is a bit much, though, unless you are also suffering from depression. It would make more sense to go with a very low dose (25-50mg) of Seroquel, or even a course of Trazadone, which is an old-fashioned tricyclic AD that isn't very good for depression, but knocks a lot of people out. Neither Traz nor Seroquel have real WDs, and while Seroquel can be a bitch for weight gain, it usually is OK in very low doses and if only used for a short time.