Sorry for the super late response here!
Jason and Jim gave you basically the best information possible but I figured I'd chime in too!
I'll start with the Sandtrooper vs Stormtrooper bit... WHICH I TOTALLY HAVE NO BIASED OPINION ON...
No really, I don't. I opted for the middle ground of Heavy Weapons Trooper. Most people use this as an addon to their Stormtrooper, but you can totally make it your primary costume too! Going HWT or TD doesn't necessarily require any additional skill, just a bit more time and money to buy the extra bits. So really, it's more down to your preferences. I've seen many people that start building their Stormtrooper and mess something up while cutting and decided to finish the kit as a Sandtrooper or HWT so they can hide their mistake. I played all my screw ups off as "battle damage"

Now kit vs completed. Go with a kit for sure! It can be frustrating going through the build process, but it can be more frustrating to tear apart a completed kit to make it actually fit you. Plus, it's super rewarding being able to say "I built this all myself" when people ask where you got your armor! On top of that, you'll know the ins and outs of your armor so repairs are super easy.
I totally get being nervous about screwing up your kit. This was the first costume I ever made and I have (had?) ZERO artistic ability. I doubted that I'd even be able to get my armor finished, but here I am approved and having just reached Centurion level. If you take your time, plan ahead, work carefully and have a little ingenuity, you'll get through it just fine and you'll be able to fix any mistake you might make. It might take you a little longer to get things done, but the end result will be worth it.
As far as kits go, like Jason said, it's hard to suggest one over another. My kit is Anovos based but I had to make some hardcore modifications to make it fit me (I'm way too tall/fat for their kit). Make sure to take a look at FISDs list of approved armor sellers (It's somewhere in there). The only wrong choice, really, is to get something sketchy, stolen or subpar.
I wouldn't worry too much about ABS vs HIPS vs PVC. ABS is definitely easier to work with in my opinion, but you should probably choose a kit based on other factors.
I know I really didn't answer anything that wasn't already said, huh?
