to be honest, i realy dont think AutoCAD is the way to go for a Tech Illustrator, in 15 years of doing this job ive not met a single person that uses it for Illustration (naturaly there will be acceptions).
With Autocad you will be severely limiting your creative freedoms. you will not be getting the high end rendering or animation capabilities of the mainstream "Illustration" 3D packages out there (C4D, Max, Modo etc).
Have a think about the future,
who will potential clients be?
who will potential collegues be?
will i be able to exchange data with them?
what kind of data will i be able to import/export?
can i use any external renderers?
do i realy want to invest so much time and money on software that only i will be using?
i think autocad is around $5000, for a fraction of this price you can get one of the above mentioned 3d softwares that is full of all the functions that a technical illustrator needs. with autocad your going to be paying for a whole bunch of stuff you will never need.
As for 2D adobe illustrator is more than capable of cover anything a technical illustrator need to do.
if you have any questions about the pricing of CINEMA 4D and its different packages or some one to one advice get in touch with MAXON inc at:
MAXON Computer Inc.
2640 Lavery Court, Suite A
Newbury Park, CA, 91320
USA
Telefon: +1 877-ANIMATE
E-Mail: info_us[at]maxon[.]net
and tell them Glen johnson sent you, they will be able to help you out