Welcome!
For background, I'm not from the US and don't have access to any local sellers. I ordered my first seeds in 2021 from a seller that was at least in the same country and some from abroad via Etsy. My oldest seedlings will be 3 years in a couple of weeks, so I am still new to this.
I have spent about $160 on a total of 97 seeds, but it should be noted that much of that cost is for Socotranum and Arabicum seeds that cost $5 each and turned out to have very low germination rate (0-25%). The ones I got from Etsy were around $1 each and had a 100% germination rate.
As a result, I have 48 seed grown Adeniums and 4 cuttings.
Of the 4 that have so far bloomed from seed varieties the sellers claimed would have double or triple petals, two have had triple petal blooms and two single.
Adenium seedlings are not true to their parent, meaning that even if the seed comes from a hand pollinated mother plant in a very controlled environment, it might turn out very differently. So unless you buy a plant in bloom, you can't be sure of what you're getting until it's grown up a bit.
Not all of my seedlings have bloomed yet, but of the ones that have, no two have looked exactly the same (the ones that are the most similar in shape and colour - single petal pale pink flowers with darker pink edges - are the ones I bought as Adenium Somalense, and they are also the strongest growing ones).
I hope this answers at least some of your questions.
Adeniums are extraordinary plants and I hope you will come go love them as much as I do!