iMac Custom Set up advice

Hi all,

I'm finally getting an iMac to replace my PC.

I am customizing the 27" and have a few queries:

Is fusion drive only available at purchase? or can you pay for them to put an SSD in later (probably cost a lot). I'm thinking I should get this, 1TB hdd is average but I assume I can update that later.

Definitely getting the upgraded graphics card as that can't be installed later.

Now for those two upgrades the trade off might have to be the CPU.

I think 3.6ghz is fine but I don't like the fact it's an i5 and not an i7.

I'm going to leave the ram at 8GB because that is a cheap upgrade.

At the moment I'm a student, most probably looking into 2D work but that could change to 3D (Maya, ZBrush) etc.

If anyone would like to drop some advice, that would be appreciated

What do people think about the processor difference?

I check that link and read an article.

Am I correct in saying that the i5 is a dual core and the i7 is a quad core?

To me it seems like the upgrade to i7 is a must.