Best advice I could give, if you feel you must go prebuilt, find a reputable, recommended shop in your area. Get an idea what you wish to spend, maybe go to cyberpower or others, get an idea what the system with the components you want will run you. But a reputable shop will be able to give you advice.
Allow the shop to build it for you, see if they will help you select parts whether they sell them, buy them elsewhere, etc. But you know if you have problems, you can go back to them for help. And they should take the time to test it to be sure things are in working order. Maybe even check with guys on here to see what a good build looks like(guys here will show you newegg.com and tigerdirect.com prices), if going local, those prices probably aren't to be expected. But I know with custom builds, some guys have local places they go to get parts, so if there is a problem with parts, they may be able to swap them back over quickly.
You might consider building yourself if you are comfortable to save some $$.
*Edit*
Keep in mind also, local shops have a reputation, and depend more on their customers than companies like cyberpower, etc. Meaning if you have problems, they are more likely to be of assistance. But larger companies might tell you tough luck.