My question is whether I should buy memory from a companies such as SanDisk, Lexar, Kingston etc. or am I okay buying my memory card off of eBay by some generic companies such as the following
AData is not what I'd call a generic brand (no high end, either), but buying cards on eBay has a veeery bad reputation. It's mainly due to the amount of fake cards (rebranded turds).
As the same cards is cheaper by
a more reputable dealer, I wouldn't hesitate (at least if I were in the USA, which is not the case - you're not either, and as such won't be able to buy at US prices ).
Another question is, considering I will be shooting mainly in raw (and probably raw plus JPEG), how much memory do you suggest I get? 4 GB, 8 GB, 16 GB, 32 GB? And how much of each one?
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No more than two cards (or you'll drop/lose/format one...), and I'd say the bigger the better - you may get a thousand frames on a single 16GB card.
I generally last a week with that, but some others shoot more in one day - your fileage may vary, of course.
If you're concerned about data security, get a backup PSD as said by DarkPenguin, or a notebook, and always keep two copies (CF+PSD). I feel though that flash cards are the most reliable data support we photographers have (the best might probably be now to add a PSD or notebook with a SSD drive).