Assuming you are planning using Unraid for your OS, (Unraid uses a JBOD setup instead of RAID) then yes you can and probably should just start with the amount of storage you need now and expand as necessary.
You could start with just 2 drives; 1 drive for data and 1 drive for parity. The one rule you need to keep in mind though is that the parity drive must be the same size or larger than any of the data drives. So if you are considering getting some larger drives down the line you might want to start with at least 1 large capacity drive as your parity so you don’t need to swap it out later.
You will need to decide if you want to get new consumer drives or used enterprise drives.
New consumer drives will cost a lot more, and have less endurance, but will have a warranty and potentially data recover service, which you should not need if you are doing proper backups.
Used enterprise drives have better endurance, but by the time they hit Ebay have probably seen heavy use. They are however much less expensive to the point where you should be able to buy an extra drive in case one of them does die and still save quite a bit of money.
A good resource for used enterprise drives is Rino Technology Group on Ebay. They are a member of Serverbuilds and will often extend deals to other Serverbuilds members. Unfortunately it looks like the deal thread has not been updated in a while, but if you reach out here or on Ebay they can probably help you out.