SAS RAID Controller help

Hi there

I have dell 7910 workstation with with SAS RAID Controller. I am using a resolve for color just a beginner My major bottle neck is my hard drives i run a speed test and i am getting WRITE 246 READ 250 in raid 0 software raid under windows 10. I am manly working with 2.5k raw cinema dmg but would like to prepare my workstation for 4k raw cinema dmg. Since i am on a budget what would be my best option be for fast raid editing 4k raw out of new BMPCC. Since my comp has SAS raid controller I am thinking purchasing 4 sas drives 1tb each of ebay for 80 dollars each like Dell COMPATIBLE 3TB 6G 7.2K 3.5" SAS DPTW9 0DPTW9 HDD Hard Drive but i am not sure if i am baying right drives and can i buy any sas drives? Or just purchase raid card with external box for HD but again it gets expensive fast my budget is 500 Other option is PCIe x16 Gen3 card that supports up to four M.2 NVMe but that gets expensive to.
And what SSD do you guys for caching files

Any help is appreciated thanks guys

My system
2x Xeon 2.4g GHZ max turbo 3.2
Memory 32 Gb DDR4
Decklink Mini monitor
GTX 770 for resolve
Quadro 5000 for 2 monitors
2 WD black HD in software raid 0 each does 64mb DISK SPEED TEST WRITE 246 READ 250
450gb SSD for os
Power Suply 1300W PSU
4 x PCI -e3x16 slots
SAS raid controller