Do you think there is any possibility of hardware backdoor, or any leftovers from Windows, that would put my bitcoin at risk? I plan to use it as my main, home workstation.
There is a huge difference between it being possible, and it being likely.
I'd say that, unless you're specifically targeted, the chances of someone tampering with your hardware is extremely small.
Although i also doubt someone would be able to "hack" a processor to spy on you, even if you were to be specifically "targeted". I think it has happend/is possible, but then we're really talking about government operations 95% of the time.
any leftovers from Windows,
This would be more likely, and much easier for someone to do. But a simple factory reset where you override the previous partitions really should be sufficient. Except of course if the previous owner of your harddrive has worked at the NSA and knows how to create such a persistent virus. -- aka: extremely unlikely if not impossible .