I would recommend buying the DE-10 Nano directly from Terasic. They're usually in stock, unlike other vendors. The shipping was $35 but it arrived in 2 days.
It isn't necessary to "have everything" to enjoy MiSTer. Besides the DE-10 Nano you need SDRAM, so buy the 128MB SDRAM 3.0 from a reputable vendor. It should be around $60-70.
At this point you will have spent $310.
Plug in the power supply and a USB hub. Flash the MiSTer software and ROMs to a µSD card. You can now enjoy most of what a MiSTer offers simply by plugging it into a HDMI display and using a USB keyboard, mouse, and game controller.
If you decide later that you want the other stuff to make it a "full MiSTer" setup, you can buy it at your leisure.