Precision 5820 - question about configuration, no. of hard drives SATA or SSD, RAID configuration,++

Hi! 

Trying to order a new 5820 Precision online but ran into problems selecting drives. Would like 3 drives, two SATA each 1 TB conventional hard drives and 1 SSD. The two conventional should be connected (RAID 1 disc mirrowing). On the SSD I would put software for programs that need a speedy drive. hard drive 1; SATA 1 TB - hard drive 2, SATA 1 TB and SSD 3, 256GB PCIe or SATA. Which kind of SSD should I go for? PCIe or SATA. ..Class 20 or 40?

Also. which drive is the win 10 installled?

Which operating system (boot) drive can I pick and which hard drive controllers do I need.? Can I trust the Dell build site to choose for me ?

Also input from Dell employees are wellcome.......