Hi roberto. I've been in a similar boat to you re which imac to choose. I have had some great advice here, some in an applestore from a confident source and from spending my fair share of time on the net.
If you look at the website of "the videoguys" and look at pc specs for video editing you'll see a great overall write up. They mention i7 is the best option for professionals. If you are not that and you can wait a while more for importing, exporting and rendering then they have the i5 as being fine and a min requirement. They also talk about ideally having a min 1gb vram.
On a budget, it appears to me that the the stock 3.2ghz 27" is well placed/suitable.
It:
avoids the 5400rpm cheaper imacs,
has owner upgradable ram,
a strong 3.2 i5 quad core,
and a graphic card quite a bit better than the entry point 27"
The card is also 1gb vram.
I think the i7 is more of a luxury item as the cores (most of the times will not be needed by us, as I see it IMHO, except for times mentioned above which will only require a little more waiting - is 5-20% more waiting ONLY at these times that important for you? That's for you to answer.
Also the 2gb vram 680 card seems like overkill for our needs. I think IMHO that the 1gb will be quite handy and very beneficial ateast for the next few years and you really can't expect to future proof for too long a period.
I too have always had concerns for the fusion drive,and question benefits for video editing. I'm waiting for feedback over next 12 months.
Another thing I read was a view that with the release of the imacs the i7 is likely to be outdated not too much after the i5 gets outdated in the future (some years away) so I don't beige we should look at the i7 option as a future proof benefit as opposed to the quicker processes mentioned above.
I have gone back and forward with my selections (changed about 4 times) and tonight I am confidentimac video editing options I am sticking with the 27 inch 3.2. I could have gone $200 cheaper but that 1gb vram sets me up very nicely for the next 2-3 years and probably pretty handily for a couple after that. If not, if i need better specs and my uses and needs change, then there will be no dramas selling it after 2 and buying again.
At the end of the day the cheapest imac will video edit (slower) and when on a budget or just wanting to buy smart the top end is overkill IMHO (again)
Good luck with whatever way you go