First of all, a new PC is always exciting, personally I much prefer building everything myself but as you've said, you prefer a professional installer - which leads me to the first point, don't go to Best Buy... or any other non-PC specific stores, regardless of them having a dept or not, they won't put as much accent on quality of the build and the employees might not be as well trained.
As for the thermal paste, just to nip the potential idea in the bud - don't use thermal pads, and you would only need to replace the thermal paste already on the cooler IF it's been disturbed in any way (fingerprints, scrapes, oxidation marks etc) but beyond that, you should be good to go. I run a TR 2990WX and I only went for a high-end paste for peace of mind.
Hope it helps!
