I wouldnt even bother making a half-life mod, why do you think that the esf team has a, uhh, well... Team? If you are serious about this then you would probably need a team that is somewhat dedicated to making this mod of yours. In my opinion, making a mod is at least 40 times worse than trying to build your own laptop, or something seemingly almost impossible like that. But if you do want to go through with this then be prepared and very very patient on making the models, maps, sprites, ect... your self. It is a pretty big undertaking if your doing it all alone, so just be prepared and knowing what your supposed to do helps out alot too.