For thermal paste, you can look up what performs well and go with your budget, but I like Thermal Grizzly's Kryonaut paste. Performs as well as Arctic Silver 5 but is non-conductive, so no need to worry about accidentally shorting something from accidental die/component contact.

I hear good things about Arctic MX-4 and Noctua NT-H1 too, so whatever you can find to best fit your budget. At that price range, differences between paste are 5c at most, which isn't a huge deal unless you're really pushing your cpu as hard as it can.

For testing software, you could try Aida64, Prime95, and cinebench r15/R20 for CPU/whole system, and Unigine Superposition or Cinebench R15's OpenCL run for GPU/whole system. For gpu thermal torture test, Furmark will make your GPU run hotter than anything else, and for CPU thermals, do Prime95 with the small FFT option.