A couple of suggestions....
A lot of people buy in huge quantities. Be careful about doing so yourself. Figure out how much you actually vape. 30ml bottle per week = 1560ml per year. This is important because PG and VG do have a limited shelf life, as does the nicotine solution. Buying a full liter of nic solution plus a gallon each of PG and VG is enough to make over 11,000ml of juice once you add in flavorings. If you're only vaping 1560ml per year that's 7 years worth of juice, but the PG and VG only have a 2 year shelf life. You could end up pouring half to 2/3rds of it down the drain because it's gone bad before you got a chance to use it.
So my first suggestion would be to buy in smaller sizes until you've got a better idea of just how much you need and how much you can store. Start with around 500ml of 100mg nicotine solution and a quart each of VG and PG. That will be enough to make right about 1200ml of juice once you add the flavorings, with around two thirds of the nicotine solution left over. From there you can judge how much you'll actually use and how long it will take you to use it all. It's also a low enough amount that you can be certain you'll use it all long before it starts to degrade. In the mean time learn more about the different methods and requirements for long term storage so if you do buy a much larger quantity in the future you'll know how to make it last as long as possible.
I know several people who have a half dozen 1 liter bottles of nic solution that think simply shoving the bottle in the freezer will keep it good indefinitely. They're in for a heck of a surprise when they find out that nicotine solution oxidizes and degrades in the freezer. It will be at a slower rate, but it's still going to happen.
My second suggestions is to head over to the DYI section here and do a LOT of research on flavorings and potential health hazards, particularly with flavorings with a butter flavor added. Be wary of flavoring companies that don't disclose what's in their flavors, avoid artificial colors, and always watch out for the hidden cost of shipping. I was tempted to buy a few Capbella flavors until I got to the final stages of checking out and found that they were going to charge $13 in shipping on a $12 purchase that weighed less than a half pound. Also it helps to know what flavorings you like and buy those in larger quantities. Some flavorings can be mixed as high as 20%, so if you're buying a little 15ml bottle that's only going to make around 75ml of juice. It's fine if you're wanting to experiment with a new flavor, but when you're talking about $3-6 (plus shipping) in flavoring per 75ml that can add up to another $100+ in flavoring for that initial nic/pg/vg purchase really fast.
