Yes, you can do that without making a mod (skins anyway).
Yes, you'll need to do some programming to add new weapons.
Is the 2009 code available? I thought the latest was still 2007. Which tutorials are you referring to?
You can use VTFEdit to convert images into files that the game will recognize. To replace the skins you put them in an appropriate folder in steamapps/<user>/half-life 2/hl2/materials/models/
If you replaced the zombie skin for example, you'd put your new zombie_classic_sheet.vtf in...