I'm new at this also, but I'll fill you in on what little I've found out.
Nic:
Some of the suppliers source is out of China, one I know says their source is the US but I'm not sure if that is on all the contents or certain ones. Best I can say there is to read on their site and if it's not found then email them or post to them if they are on the forum. I've heard of one case where when a product was tested it didn't have the amount of mg in it that it was being sold at, thus seeing if the product has been tested and reading what others who have used it say would be worth while. You definately want to buy nic that has already been cut with PG or VG, doesn't take much of the pure uncut nic on your skin to do great harm or even kill you.
http://www.e-cigarette-forum.com/forum/diy-e-liquid/39124-99-nic-juice-5.html
PG: Needs to be USP
Has antibacterial and virucidal properties, it doesn't need any thinning with distilled water or alcohol. Some have had reactions to it so they don't use it. It has been used in making illegal drugs so it is behind the counter at pharmacy's and they tend to ask what you will be using it for. Just look for PG in the topics on different pages or in your search and you will find where/how some are getting it locally. Some of the retailers in the forum sale it and other locations on the net that pretain to making cosmetics, soaps, etc. sale it.
VG: Needs to be USP and most say pure vegetable glycerin is best (not made out of animals, etc.)
VG provides more vapor, it's thicker so it needs to be thinned with distilled water, alcohol or PG so that it won't clog the atty. As you said they say it adds a sweet taste. Some have had stomach problems using VG. Heard it can be found in the cake isle if it's vegetable oil, or in the pharmacy area next to skin treatments (says external use, but it's USP and what some are using).
Alcohol (vodka, etc.) used as a thinner provides more of a throat hit. Not sure exactly what that is but if its the burning effect I don't care for it myself.
I don't seem to have a problem with PG or VG so far in the juices I have bought. Some have more vapor thus I know they have more VG in them. I prefer having more vapor and a good flavor. I'm not sure where the cut off is (think it is in the calculator thread below), but I do know with 80% PG and 20% VG you wouldn't need to thin the VG so that will be my starting point.
Flavors everyone tastes differently, some are stronger than others though even from the same company (may even be an effect in different batches of the same flavor), but the below link gives you an ideal of what people have thought about Loranns and what strength to use (color coded list tells what not to use and which has dye in it which we get everytime we eat meat and a lot of things anyway). Main thing on flavors is to make sure they don't have sugar or oil in them or additives that are harmful. ie: oils, vanillin, or diacetyl
Another thing to consider is storage and shelf life of your batch you make or buy and your ingredients. Most say your nic if stored right can last up or over 2 years. Juices some say 2 years if stored right, others say 6 months or less. As for the PG, VG and flavors they don't have a long shelf life either (not sure exactly how long) so I plan to only buy those on a shorter term usage. A bill before Congress is out to ban the mailing of tobacco products, thus I'm looking into buying 2 years worth of nic so my equipment can at least have paid for itself.
These articles have come in handy for me when it comes to ingredients and etc.:
http://www.e-cigarette-forum.com/fo...e-calculator-added-quantity-calculations.html
http://www.e-cigarette-forum.com/fo...rs-color-coded-complete-ingredients-list.html
http://www.e-cigarette-forum.com/forum/diy-e-liquid/12259-loranns-strength-o-meter.html
http://www.e-cigarette-forum.com/forum/diy-e-liquid/45736-sterilising-tools.html
http://www.e-cigarette-forum.com/forum/diy-e-liquid/24324-recommend-starting-equipment-kit.html
Searching and digging is like pulling teeth here. At least when you wade through all the pages the word you searched for is in red. Any more questions hopefully someone else can help you with, if not I'll check back in a couple of days on this post.
Editing-typo's and misslicks
