The brand of flavor doesn't matter. Every company that is out makes a good flavoring and usually have that 1 flavoring that other companies counterparts just don't compete with. One thing I will say on this subject tho don't be like me. I used to skimp out of "expensive" flavorings with that I missed out on a ton of great ones. I recently did the math to compare expensive and cheap flavorings price per ml and percentage used and most of the time the expensive flavoring actually ended up cheaper compared to the cheaper counterpart. Again you shouldn't limit your decision to "expensive or cheap" going back to every company makes good flavorings. Now this goes into your example of orange creamsickle. You could probably tweak any flavoring to work in favor to create what you want it to be but there are flavors that make that process simpler. Doing research is going to be your best friend with these but nothing compares to getting first hand experience with a flavoring.

Nude nicotine isn't pure and is diluted. Any reputable company isn't going to sell you pure nicotine without the proper certifications on your part. So you'll be safe on that note. I would shoot for something lower on the ml/mg range just when you are first starting. For me my personal favorite is Nude Nicotine Nude Armor V2.

Lastly on Nicotine purchase a nicotine test kit. Something every DIY'er should have and be familiar with. Here's a video I found on the fly https://www.youtube.com/watch?v=wWEcNwefUhw