You will most likely get varied results using preground coffee.
Even if you seal the ground coffee in an airtight container and open it only prior to using it, the coffee will age quickly and go stale, giving inconsistent results.
Is there someone that you know with a grinder that you could trial with your machine?
Your concern about "having" to spend a lot of $ on a grinder is right and wrong.
You can get something like a sunbeam 450/480 for circa $120 - 180 I think, and that will "do the job" and get you better results than preground. If you can afford it, spend more on the grinder and aim for something like a Rocky / Compak / Ascaso etc.
It has been said on here before that coffee once ground starts to deteriorate after 3 minutes, that figure may or may not be right, and you might be able to stretch the time out a bit by sealing it in an airtight container, but fresh ground beans used within the 3 minute rule will certainly get you better coffee.