Mercury is important because the computer is basically dealing with information. It is also a tool to extend your mind abilities.
Uranus is also important since it rules the electronics.
Saturn has nothing to do with this. Mercury and Uranus are the natural significators.
You are right when you said that a fixed sign rising is very important. Even more, if you can adjust the time so that the Ascendant will be as close as possible to 15.00 in any fixed sign, it is even better, since the 15th degree in fixed every sign is surnamed the degree of eternity. This way you'll know that the computer will work forever...
Make sure that the Ascendant ruler is not retrograde. Elect a day with good lunar applying aspects.
The house for the articles that you buy is the 2nd house. Try to find a moment when there is an applying aspect between the 1st and the 2nd house rulers.
For more information, read the Electional astrology article recently published.