I have two issues I don't know how to tackle, as follows:
Occcasionally I need to select a group of connected lines, merge them together using Control+8 (Windows); select Object, Path, Outline Stroke; and then apply a solid black stroke of 0.15 point to the selection. I may need to do this a dozen times or more in certain open documents, and it's very tiring to do it all manually. How can I automate this?
Second issue: I use a specific "arrowhead" all the time - flat bar (arrowhead #27), at 40%; sometimes at the beginning of a line, sometimes at the end. Is there a way to set this default value?
Any combination of things you can add to an object in the Appearance window can be saved as a graphic style, so yes, brushes too.
Add graphic styles to the palette, select another object, and click on the graphic style to apply those appearances to the object.
You might want to save graphic styles as a library for use in other documents (see the drop-down menu in the graphic styles palette), or save a document as a New Document Profile if you want to use those graphic styles in all new documents. See here for more info: