Does anyone have experience here creating a source mod?

From what I've seen, source mods are a glorified map collection. Simpler concepts, such as telling a story entirely within the Half-Life universe, don't even require custom materials. They can still be extremely successful: look up the Half-Life 2 Minerva mod. If you want anything else, you have to make it yourself. New models, sounds, voice lines, and so on. More complex examples would be like the Apeture Tag Paintgun Testing Initiative for Portal 2: ask Motanum about it if you want additional details. That stuff can take years with multiple developers to come to fruition.

if you don't intend on modifying the code, I'd advise to just make maps for hl2 and worry about making it a mod later.

Well, I have created more than a few Source modified games in the past, but the finished product was always a test of a program concept I wrote up in C++, but I plan on releasing one in the future.

You need to first determine if the mod will be singleplayer or multiplayer. The entire mod's internal structure revolves around this choice. Single-player tends to be much easier to develop for, requiring little to no code, besides copying and pasting files and changing certain variables and names. Multiplayer, on the other hand, requires an entire different outlook. Much less documentation is on it, requiring you to have basic prior knowledge of how the Source Engine works and its programming.

Do you have any programming knowledge?

This is a good place to start

For documentation on mods ing general and everything you'll ever need, look here

Good luck and I wish success in your endeavors.