Your qualms about building. Did you find out how much the tech would charge you? Would he be willing to oversee you doing the build?
What monitor will you be using? and does it have speakers?
Although it was my good colleague iammykyl
that posted the question it may help if you answered it, so that he may provide the best advice
If it is a CPU boxed deal with cooler it will come with thermal paste pre applied - usually
Some people, especially if they are going to overclock, not something I would recommend, prefer a more specialised application of thermal paste, than is provided as standard
What the thermal paste actually does is to cater for the minute imperfections as such in the surface of the CPU and the cooler., where they mate together.
FANS it depends on what if any are provided in the case, and what is not.
Basically you have the CPU cooler fan, then you have a fan drawing air in and a fan sucking air out.
These if fitted to the case, will be designed so that a balanced air flow is achieved
If the case only comes with one fan pre fitted then yes you need to consider that.
We are really back to how qualfied this person is you are employing , for want of a better word to assemble it, if he is taking responsibility for the correctness of the build, then I suggest it would be foolish not to follow his advice., as of course if you do not, and the system overheats then - where do you go from there.
As you can see, my good colleague phillpower2
and I posted together - and as you can see - we agree.
We can I feel certain, better advise you, when you tell us, unless you prefer not to, some details of the arrangements for this person to build the system.