And while I am new to the forums, I am not new to acrylics..... The quality of the brush painting is going to depend on the quality of the paints, but it is possible to paint large models in acrylics without an airbrush. It requires a good primer coat, and thinned paints applied in multiple coats. I will try and throw up some examples of stuff I have done (although they will be OT models...) You will not have the workability that you did with the humbrols, but depending on the brand of acrylics you go with, you should have decent coverage. I personally use GW acrylics, and have friends who swear by both Vallejo and Reaper master series. I have also used Tamiya and model master. The tamiya does not apply well with a brush (the thinning agent is too effective IMO and attacks the previous coat) and I the model master acrylics didn't really leave an impression one way or the other.
So, yes, you can brush paint with the acrylics, but it will take significantly longer to achieve a solid smooth color coat than if you use an airbrush.
-Adam