Everything is compatible.
Looks to be a fair deal (the anti virus is a cost and half the discount free ones like avast are usually enough).
HD6850 or GTX460 1GB at minimum.
Size wise fine, brand is hit and miss.
Maybe build the basic rig in shopping cart without the Anti virus and with a known quality PSU and see if you can get similar discounts through other combo deals.
1) Yes, the parts are compatible.
2) I wouldn't say it's a good deal. You have to purchase the Anti-Virus software (which I wouldn't use) and to continue to use it, you have to pay an annual subscription fee (after the first year). Additionally, I'm not crazy about the case. It only comes with one 80mm (would prefer a 120mm) rear exhaust fan and a side vent. It doesn't look like there is a place to install a front intake fan (also would prefer 120mm) either.
3) That depends not only on the games you want to play, but also the resolution of your monitor. The higher the resolution, the better card you're going to need for smoother game play. What resolution are you currently using?
4) I can't answer this. I don't know the quality of HEC power supplies.
5) I'd probably pass on this combo. I don't like the case and I don't like the must purchase software.
All parts are compatible. The combo is OK, not great.
You can run the iGPU on the 2500K to start and get a discrete GPU card later.
The hec 600W is not a prime quality PSU. It is not even 80 rated.
If you are willing to take some suggesstions I'd recommend that you keep the CPU, HDD, mobo, and RAM. Maybe select alternative mobo or boost RAM.
Dump the antivirus, case and PSU. If you get Win 7 then you can download MS antivirus. Get a better quality PSU (even if lower wattage), and a better case. The case is important for cooling and your rig will run hot in that case.