If your own area of the site is intended to be completely stand-alone outside of mojoPortal, you probably won't need the full source. But if you do that, it will be really difficult to maintain a consistent look and feel for the mojoPortal and custom parts of the site (if that's important to you). If your needs don't extend as far as an entirely separate application, you could do what we have done, which is to develop custom features that run within mojoPortal (and can be placed on pages just like any other delivered feature). Doing this doesn't involve touching the base code at all (which is one of the great things about mojoPortal). If that will work for you, all you need to do is go through the mojoPortal Developer Documentation track, and also watch all of Joe's great videos. An example of a custom feature we developed is here: http://www.escondido.org/parcel-lookup.aspx.

If you decide to follow this recommended mojoPortal development method, you should definitely download all of the source and keep it current with TortoiseHG as you're developing. I just ended up creating a subdirectory off of the root of the mojoPortal source folder, (e.g. _custdev), where I keep all of my source. This is the spot that's recommended in this developer document. We have at least one additional developer at our site who will be working on mojoPortal features, so I plan to explore setting up our own internal source repository on this directory in the future.

I hope that's useful.

Jamie