Your situation sounds very similar to mine! I had my colon out 7 years ago, but had my Jpouch made in May with tempileo and my takedown 3 weeks ago. I am a new guy here too.
I had a fluoroscopy exam before my takedown and was very nervous and there was no reason to be. I couldn't eat or drink after midnight that day. They insert a very small tube up your bottom and then liquid from an IV like bag flows in and out with gravity. Completely painless and fast, Hardly can feel it in there.
In terms of post takedown, I have had fantastic results with a bidet. It is a brondell from Amazon for about 40$ that attaches to your toilet. Everyone on this forum raves about calmoseptine and I didn't find it as helpful advice as others. In my experience when you are going often, if you clean yourself, put on calmoseptine and then have to go 5 mins later (which will happen a lot) the poo and thick creme get mixed and then your skin is sealed in with moisture and acid. I initially tried this method and had serious butt burn. The second I swapped to the bidet, it was healed in a day. If you go, completely power rinse your whole area, and pat down with Charmin Ultrasoft (I sometimes even use a hair dryer on low to fully dry) it helps preserve the skin and lets it heal. I haven't had a problem since day 2 when I made that switch. The key for me was clean dry bottom. Obviously my experience and opinion, others have had awesome success with creme, so I would buy both and see what works for you.
I got Depends adult diapers that I have been wearing at night and on certain occasions, just in case. I have had tiny leaks here and there and it is good to have the confidence of the diapers.
In terms of weight gain, I have always been a skinny kid with really high metabolism. I was 135 when healthy and dipped to 115 by my takedown. In the three weeks I have had my Jpouch, I am already up to 125 which I can't believe with my skinny body. I am hungry all the time and I have been chilling this month close to home literally eating all day. I have been doing shakes with boost plus, peanut butter, bananas and avocados, and other high calorie foodstuffs. If you eat enough calories, the weight seems to come back.
The two things I will say about eating is fiber and gas. I was initially constipated and straining all day trying to get stuff out. The fiber works miracles when you have diarrhea or constipation. Simply stabilizes your insides and let's everything flow nicely, I think it is completely necessary to have. For gas, I was initially having a ton of gas that was very uncomfortable and thinking it was from food like dairy, but my doc told me gas is made in the colon, so the only gas I have is from me swallowing air. Really try to focus on eating slow and not gulping. I was always inhaling down massive portions of food, and I am working on eating slow to reduce gas swallowed. It makes all the difference.
Lastly, do your Kegels. I wasn't using my bottom for 7 years because I had a end ileo, but now that mine is back in use, they are tired. I have been fighting some nasty cramps right in my anus, and doc says these are from my muscles getting used to having to hold again. The stronger you get the pelvic floor muscles the happier and less cramping you will have. Expect 4-6 weeks of discomfort in that department.
Overall I have had a breeze of a transition in my experience. I am surprised at how good I feel.. Im not meaning to brag but rather give confidence. I think hearing a good story is reinforcing since most people talking on this forum are experiencing tougher times. The biggest piece of advice I found on here was stay positive and know and accept that the month after (maybe longer) is gunna be kinda sucky. If you can go in with the mindset that it is going to take time and be terrible, it doesn't seem nearly as bad on the other side. I wish you the best of luck and hope everything goes well!!!