Typically, spindles for woodworking are not designed for heavy axial loads, like drilling. You shouldn't have trouble with large diameter tools, but cutting steel may put more force on the bearings than they are designed for. I have no experience with the chinese spindles, but these are recommendations for more expensive spindles, like HSD's and Columbo's. They recommend not even plunging into wood, but ramping into the cut.
I think you may find that at slower rpm's, the spindles have much less power. I don't think it'll hurt the water cooled ones to run them slower, as long as it has enough power to do the job. I know it's not quite the same, but I've used a 6" diameter raised panel cutterhead on an HSD 10HP (22K rpm, electric fan cooled) spindle at around 7K rpm.
Just realized that you want to run at 3K rpm? Not sure about that at all. ???
This .pdf might give you some info, although it's for Columbo spindles.
http://pdscolombo.com/pdf/PDS%20Spindle%20Care%2007.pdf