Hello - I'm still new and despite reading previous threads some of my questions are unanswered.

I'm thinking of transitioning my bitcoin from an online exchange site to electrum to be able to benefit from upcoming hard fork. Previously this said exchange site just swallowed the new currencies and I'm fed up with it.

Hence, I plan to install the watch-only (online) + cold storage offline wallet combo from Electrum. Since i have never done these I have a few things unclear:

0) I want to keep my BTC for looooooong time without transacting - just like a savings account. Is the above combo ideal for my needs?
1) will i need to set the seed (passphrase) to both watch-only and cold wallet? Do they have to be the same?
2) I keep reading about how important backups are. I assume I totally need to backup my offline cold storage. But does it make sense to backup the watch-only wallet too?
3) When I install these wallets how much GB they will take up roughly? I have an old but still reliable machine with limited storage so wonder if I hit a ceiling
4) when I transfer from the exchange site, do I need to "receive coin" from the online wallet or from the cold wallet? Where do I send it to?
4.5) I understand sending funds needs to be signed by the offline wallet. But do I have to sign received funds too?
5) Does the online wallet need to be online most times and would it generate all kind of constant internet traffic in the gigabytes per month?
6) in the future when I want to sell it,  how do I transfer it back to the exchange site?

Cheers
Snailrider