I think you already answered your question. If you don't want to build anything, then your options are major manufacturer based. If you choose to go this way, you will most likely be faced with the need to add a SATA card and it may not support the amount of RAM you choose or the OS options are outrageous, as most cookie-cutter machines are standard issue. You will also face a large cost for purchasing a machine with what you need for RAM and disk requirements, as these machines tend to be "developement machines" and have a higher associated custom cost. You will end up with software, and possibly hardware, that you don't need or won't use.
My suggestion is to just build one. You get exactly what you want and, chances are, that you will come under any price a major line would offer for a comparable machine.