Instead of putting the money into a snake that wont actually do anything to help you why don't you try to find a reliable mentor and learn the right way?
There is not a single non venomous snake that will help you prepare for any venomous snake.
Try as you might, you will never see that non-hot as a threat, thus you will always be subconsciously more relaxed than you should be with a hot. Not only that but no non-hot is going to have the same behaviors and tendencies as any hot.
Also, a gaboon viper is a terrible first hot, they are wicked fast, unbelievably maneuverable, and extremely deadly. If it was me I would never let someone I trained start with a gaboon.... They may be well known for being pretty laid back but that in itself is a terrible trait for a first hot, it teaches you to be complacent and allows you to get entirely to comfortable before you have ever really learned that that's not acceptable. On the opposite side a wild psychotic wiry hot is also not a good first in my opinion.
Point is... get a mentor, learn with the things you want to work with, that is really the only good way to get prepared for hots...