1) You can either Reinstall or use a Free Program like EASEUS Partition manager and make the Raid 1 and then Clone your current windows to it. Then remove/format your old OS drive after you can make sure your booting from the Raid 1
2) A Raid 5 is mainly for redundancy. It WILL NOT be as fast as a Raid 1, 0, 10, 0+1 due to Parity bit overhead which can also tax the CPU a bit. If it was strictly data storage I'd say go for it but for an OS or a drive with a lot of writes it not as good.
I could just stick with the Raid 1. When you clone the OS it will resize the parititon to fit. If EASEUS Don't work Use Marcrium Reflect and make a Bootable Disk. The reason why i suggest Easus first is because its better at cloning OS partitions.