I dont know where you live, but buying a PC from somebody that as one shop, will scare me. You dont know what is inside ! it could be parts that failed to 100 % test
You should look at the Apple refurb, you can get up to a 1000 $ off your Macpro ! ( I have bought my two last Macpro from refurb )
And if you take the Apple care they come to fix it in your house ! (3 years garantee an support for about 300 euro)
If you buy a PC from someone who deals mainly or even exclusively with audio clients, you will get a machine that has been tested far more for relevant applications than buying a machine from Apple. That's why the cost is more than building it yourself. You will also get much better service than you would from Apple, and your software would also probably be pre-loaded.
The only reason to give Apple any money at all is if you like OSX. If you need the best performance, your reason for getting a Mac has gone. Unless you happen to use Logic. [;)]
DG