If finances are tight, you could start really low with a 1TB hard drive and upgrade later on when wallet allows you to. At least this way, you can get started. If I ran a very small TB hard drive, I probably would not run 24/7 continous recordings but instead just motion detected or IVS detected. At the same time, it depends on how many cameras you will be using. Heck, honestly...it is not a mandatory thing to purchase a Western Digital Purple HD though it is highly recommended.
Using a small TB hard drive that is not a WD Purple and not recording continously is far from ideal and not at all recommended. However, not everyone can afford what is truly required on day 1.