New flight-sim PC - build or buy ready-made?

It's been quite a few years since I last built my own Windows PC - probably over 5 years now. In those days it was cost-effective to build a high-end PC. Having just returned to PC hardware now, I want to build or buy a new Windows PC that can run MS Flight Simulator or X-Plane 10 Global at high to maximum display settings.

I already have a flight-sim controller (Mad Catz Cyborg Fly 5 joystick). I also want to re-use my HP 27" monitor (1080p), Apple mouse and keyboard.

Hence, I just need the base unit (case. PSU, mobo, RAM, HDD, graphics card, CPU + heatsink, etc).

So, is it worthwhile to buy in the components and do another build (I'm still fairly confident about building a new PC even though it's been a few years) or just buy a ready-made base unit?

I'm also after some advice on CPU, motherboard and graphics card that will run a modern flight sim at max settings.