I lost about 4 kg on my pre-op diet, but I started the pre-op diet at 166.4 kg which is significantly more than you. There was no concern about my liver, my surgeon didn't even mention it post-op. And weight started just dropping off as soon as I got rid of the surgical fluid and gas.
DO NOT FREAK OUT if you gain weight right after surgery. They pump you absolutely chock full of surgical fluid and gas and it can take a week to pee and belch it all away.
Make sure you have a couple of follow-ups with your psychiatrist (or whoever prescribes your bipolar meds), one 2-3 weeks post-op and one about 6 weeks post-op. You may need to have your medication levels changed a bit, and if you have extended-release meds (such as carbamazepine ERC) you will probably need to have those changed to regular release, since it won't sit in your new stomach long enough to be absorbed all day. It's annoying to have to dial in meds again but it's worth it.
All you need at the hospital is your phone, a LONG charging cable (some of those outlets might as well be in another room for how far they are from the bed), lip moisturizer, Biotene or similar mouth moisturizer, a comfy and loose set of clothes to go home in, and a pillow for the car ride home in case the seatbelt presses on your incisions (mine didn't, but just in case). You can leave the pillow in the car, obviously.
Good luck!