In a 4X4 room you do not need 2 600w lights, 1 is fine. ebay or craigslist are good for finding used equipment. You can buy some of that stuff in stages. You should not need the filter for at least a month. The additional supplies you added should not cost near $1800.
You may want to start small with some bagseed to get evrything dialed in. I think you will find you do not need as much as you think for that size space. 15 plants in a 4x4 room can be done, if done right, but I would start smaller. But that is me, good luck.:thumbsup:
If you look at the link in my sig, that is a 4x4x7 room, for reference.