$350 of Shulgin books?! I didn't know he had written that many! Not sure why you would feel guilty about reading up on a topic of interest. Maybe buy the books in installments?
As for the DMT - the nerves won't go away! They are natural. Even after dozens of journeys I have to really prepare my body and mind before going in. This is one of the 'tests' that DMT presents us with imo.
5mg for me would barely do anything.
10mg would be the first glimmerings of an experience, but more of a body load than anything else. And this is being vaped efficiently, in one breath and held for 10-20 seconds.
15mg is where it starts to get visual and interesting for me, and beyond 20mg we are entering breakthrough country. This is using a GVG though, I would need more if using a bong as it is easier to burn the Spice, plus there are other factors such as water and more glass to condense the vapor on its way to your lungs.
10mg won't give you an idea of what lurks at 20+mg, so if you want to explore deeper territories then at some point you will just have to take the plunge.
If you are planning on taking the Valium to calm a trip down / abort, it will be over before it kicks in any way. If it's to calm the nerves before hand, you should look into other routes, such as meditation and / or harmalas (which also slow down and potentiate the DMT)...
Just relax and go with the flow, the Spice will take you where it wants and trust that you will be back in 10 mins, most likely with an enormous grin on your face.
Good luck!
Lose Control, Free My Soul, Break Me Open, Make Me Whole."DMT kicked my balls off" - od3