Selecting a right DVD disc

1)
How can I choose a good DVD disc?
Which brand?
What storage (GB) should I buy?
Are the storage volume the same for all DVD media for the same type?

2)
** Speed **
And what speed of DVD should I purchase?
Should I just go for the fastest one?

3)
** Single or dual-layered **
Which one should I choose?
And why?
Any compatibility issue?

Apart from dual-layer DVD, is there any better DVD media in the market now?
If so, what are they?
How good are they?

4)
** layer/structure/dye **
I'm not sure if the layer or dye used by the DVD is important.
I wish my DVD can be kept for long time. I don't wish to lose my data.
If you know about that, what layer/dye of the DVD should I buy?

Thank you