yep that power supply will easily handle those parts, and yes the bundled cooler with the 2500k does come with thermal paste pre-applied.
I would however recommend getting something like a 560 ti (just one, or even look at pricier cards if you can afford it) now, and then SLI it with another card in the future. SLIing two 550 Ti's should give about the same performance as one 560 Ti, except it carried the risk of micro-stuttering and higher heat and power consumption.
Yes, you get factory Thermal Paste. But I recommend doing it yourself and using Cooler Master Arctic Silver 5 3.5g. But that's just me. It's only about $10 shipping and all.