Assembling a computer, clarifying some issues

Hello everyone! I am assembling a high-end computer for myself. By the end of the month I buy a video card, then I will take it gradually at a downward price - mother, processor, etc. In connection with the expensive cost of components, I would like to clarify some points:
1) How safe is it to buy goods in online stores? Primarily interested in Xpert.ru and Megabit. There the prices are the cheapest for interesting vidyuhi. Or is it better to overpay a couple of thousand, but take it in your city?
2) How risky is it to take components gradually? Could it be that in six months, when I assemble the entire computer, some part will not work and it will not be changed for me?
3) Does the thermal paste tend to dry out over time and what additional parts and cords may be needed when assembling the computer case on your own, except for those that will be in the component boxes?

So far, everything seems to be))