I have a (hardware) technology idea that several companies might be interested to compete with once created. Once I apply for a patent, can I proceed to scout for manufacturers, hire engineers, etc. knowing that the patent application was done and those who copy will be too late, or is it better to wait until it is completed?
Also, how long do patents take to complete processing, in the US? And can non-US citizens obtain patents in the US?
I don't see any benefit to waiting until you get an actual patent out of your patent application. Getting a patent in the U.S. typically takes 3-5 years, but there are ways of speeding that up (for that, you need to talk to a patent attorney).
The U.S. does not put any restrictions on non-citizens, but your own country might. Some countries require you to get a "foreign filing license" before you file a patent application in the U.S., and they might make you file in your own country first.
Note that you do not have any rights until you actually get a patent. So if you release your product next month and a competitor copies you and releases the same product in two months, you can't do anything about it until you get your patent, which could be years later.