As Scott wrote, Onstep is a great way to go for full computerization. A fair bit of integration of mechanical design required to get the stepper motors to actually be driving the dob - I'll leave it to Scott or others to comment as he has a lot of experience with this. I believe some people use 2mm toothed belts stuck to wooden discs to make large diameter "gears". I have a setup which I bought from scott more than a year ago which I am (shamefully) yet to install!
Equatorial platform is quite cheap, easy if you are somewhat handy. there are many ways to build them. I have one which uses "vertical south segments" (see Rainer Vogel's website.
http://www.reinervogel.net/index_e.h...attform_e.html
Mine is motorized with an aliexpress-purchased celestron simple analog adjustable motor, (google celestron single axis motor drive aliexpress). not all that cheap at $50 but convenient if you don't fancy making a simple stepper motor drive).
I also built a much bigger one of a different design which carries my 22" dob. I call it an Abele-Fangrow platform, as it incorporates elements of the equatorial table from the AFU-75 soviet satellite tracking telescope designed by Latvian Maris Abele, and the inner-tube air escapement "hiss drive" devised by american Tom Fangrow. Power is by periodically pumping up the tube, no electrical drive or batteries to go flat out in the field! I have used it several times at star parties, provides excellent smooth tracking but only for about 15 minutes between resets. But that is good enough for star parties as it means I can set an object and a bunch of people can go up and down the ladder to observe at the eyepiece before the dob needs re-aiming.
