There are other threads discussing this that have links I'm too lazy to look up right now. Suffice it to say, that you likely cannot play most characters in a 5th edition campaign in HL using only the SRD. The value of the SRD is that it provides a framework for the community to fill in the gaps.
If you are not a do-it-your-selfer or are not not comfortable with scripting languages and don't want to deal with the learning curve of using the HL editor to add custom content and rules to fill in the gaps, I would wait a few weeks to a couple months until the community has put of the files with the mechanics necessary to create any character type.
As it is now, you will not have most of the subclasses, none of the freat (well, okay *ONE* feat), no backgrounds, and some spells are missing.
Note that when the community fills in the gaps, due to IP issues, the community files will only have the mechanics. If you want the full flavor text from the PHB, you'll need to fill that in yourself, but that is much easier to edit. It is coding the mechanics that is the challenge. Besides, you don't really need all the flavor text to actually play the character.
Other will have more details. But if you are hoping to buy the SRD for HL and be able to use it to create and play characters in 5e campaigns, off the shelf, you'll be disappointed.