Hi there. I did my one and only tri about 10 years ago. I was in a very similar position as you...generally athletic (running background), curious and up to the challenge, and knowing that there was a good chance I would only do it once. As far as $ goes, you can easily spend several hundred or even a thousand if you do what the pro and enthusiast triathletes do. I agree with everything ericb said, he has about 100 times as much experience in this as me.
Your first list is a good baseline. Your second list is optional. I'll get into bike/tri shorts next...
Trisuit, no (read bike shorts below).
Belt, no. Paper numbers are a thing for running and bike races where you can just pin it to your shirt and go. However, obviously with the swimming, tris are different. The one I went to, they wrote the number with a sharpie on your calves and arms. If they do force you to wear a number, pin it to your shirt for the bike/run. You only NEED a belt if you NEED a bunch of crap during the run. During the bike, you can store said crap on the bike.
Race nutrition, kinda. I think what you do with the alternative nutrition is great. Everyone will tell you that at the race, you do what you did when you were training. So, if you're training with honey and powder mix drink and it's working for you, do that in the real race. The fancy gels like GU and the gummies are unnecessary. Some people like them and will recommend them, but you don't NEED them and they're expensive. For your powder mix drink, put it in one or two big bottles and store it in the bike. You will have a little "transition area." Take 30 secs for a water break in the two transitions and drink as needed during the bike. The running portion for most sprint tris is a 5k. Most organized sprint tris will therefore have 1/2 water/gatorade stations during the run.
Bike shorts. This is the one thing where I'll recommend spending money for the below reasons. They do make bike shorts that you can swim in for just a little more $ than normal bike shorts. You can wear these for the entire race, then all you have to worry about is shirt and shoes. I still have my bike shorts and use them occasionally for running or as underwear for other sporty things. Go to your local bike store and ask if they have tri shorts. They will be around $50. I wouldn't recommend buying them online, as you're not quite sure what you're looking for and want to make sure you can use them for swimming also. Maybe someone else can link to some good options so you CAN buy them online. Obviously, practice running and biking with them before the big day.