This much I know. Your Standby server cannot be on the same machine. It can be on VM am suppose. But the configuration should be identical to your main server. Please check this link for an over-view on stand by server.
How many users are going to be there. whether they use fmpro or webdirect or fmgo. You have to consider all of this while getting into buying the server configuration. check this below link too.
I would suggest the idea of having the standby server on the same machine (if even possible) is folly.
The basic idea is to replicate the "hardware" in the event of hardware failure and the software in other cases. My understanding is both servers are required to run together and replication happens in the background.
In past installations of FMS13, I've always duplicated (Mac environment) the computer and thunderbolt disk arrays that in the event of either failing I had a swap available. All OS and data were stored on the arrays.
In terms of best Raid, I would recommend Raid 10. It has always provided me with data security and good read/write performance. As for Hardware requirements, depends on your deployment, number of clients, use of web direct, database size and number of databases open. The more users you have and the more hosting you provide the more cores and memory you'll need. The recommended is a good starting point, it depends on your budget and usage growth forecast as to what you ultimately choose.
The use of standby server should you choose to implement that configuration will double your cost as both machines are supposed to be identical.