Are you looking for a custom computer build and not an off the shelf unit? Maybe you looking for a particular case and lighting configuration? how about a custom liquid cooled system or mineral oil submerged system aquarium pc case?
When choosing a system you need to figure out first how much you want to spend. the next thing is how much stuff you want to try to reuse from a previous computer. You may be able to reuse the case, power supply, CD/DVD drive, keyboard, mouse, etc just for some examples. this can save money on the new build. note on the power supply that some situations the old power supply will not work on the new build. common issues with power supplies is companies such as dell and others sometimes have there own wiring configuration that does not comply with the standards. such power supplies will fry the board when powered on.
More info to be added