Well, I think is the best if you buy an "Manufacturer refurbished" machine. All in one, like Juraj wrote.
But, for the modeling you have to thinking about the Quadro ! And it's deepend what you want...Laptop or desktop ? Anyway, the best solution is, if you DON'T build your workstation.
If you want to use the Corona, than the GPU you need only for the modeling...For the rendering stronger CPU is the solution, and the DELL is the best ! The warranty for your workstation is only up to you and your budget... I thinking like this...And if you was an gamer...just forget it anything you learned about the motherboard and etc...Because that's an different world
Cheers!