The reason I bought a computer is to do my taxes. (every year from now on, perhaps) The reason I bought a computer was to do my taxes. (on that occasion OR every year from now on, perhaps)
1. If you are still doing the project or if you still intend to do the project, you can use "is" or "was", and they both mean nearly the same thing. 2. If you have finished the project or if you no longer intend to do the project, use "was".
In the first case, you have the choice of matching tenses "bought"