[exim] Advice: NFS, hardware, SATA vs SAS etc

Hello gurus,

As 2020 nears I wanted to ask opinions about the current state of hardware requirements for a small business email platform. $dayjob asked me to enhance our existing platform to improve performance and add redundancy.

Main questions (TLDR):

Can NFS handle heavy IMAP, LDA, HTTP workload?

Is direct attached SATA III 6Gb/s SSD in RAID 1 sufficient or is SAS needed?

Is gigabit ethernet the bottleneck in any case? I can upgrade to a 10 gigabit local network if advisable.

Can NFS peacefully co-exist with other mail system workloads without resource contention such as SQL DB or spamassassin or redis, etc?

More detail:

I had bad experiences with NFS approx 15yrs ago where IMAP load saturated controller link (yes, noatime was used on the mount) and was unusable. But in 2020 is it time to give NFS another look?

Current mail storage setup uses local attached large SATA SSD and does well, but it directly hosts HTTP, LDA, IMAP and Submission which could all be faster and it only does nightly backups. Adding SAN is probably out of $dayjob pricerange and SAS is borderline. Power consumption is also a factor so instead of a dedicated file server I thought it would make more sense to build a big server with direct attached fast SATA SSD in mirrored RAID that also has strong CPU and maximum memory so it can also run some of the backend process such as spamassassin, redis or SQL database etc. (we want to start using SQL DB for more which means it will be under heavy use)

What workloads can best co-exist with NFS where each does not contend for the other's resources?

I'd put a couple smaller machines in HA in front of that to proxy webmail, HTTP website, IMAP and Submission. Edge MTA is on a separate server and would probably stay that way, maybe adding a failover. It keeps a fraction of its mail in the local system but will make more heavy use of the SQL DB which I thought to also put on the file server(?)

Could NFS keep up with load for proxy of HTTP, IMAP, LDA, etc?

Is local attached SATA SSD in RAID 1 ok? Will 6Gb/s SATA III be a bottleneck in any possible scenario? I was looking at motherboards with multiple PCIe or M.2 slots thinking NVMe bandwidth (3GB/s) would be great but I'm unsure if NFS, gigabit ethernet or other components could even make use of it.

Or is that too amateur and local attached (hope not remote attached) SAS a minimum requirement? SAS SSDs are a newer thing I never used and expensive for the $boss. I read some people express doubts that SSD is suited for SAS at all which is one reason I thought just use SATA for more efficient power and cost.

Our workload:

We process a lot of mail but as you can guess, don't have tremendous storage needs. We have several tens of thousands of users but a smaller fraction of that are actively using mail every day. Maildir storage is several TB. Exact daily mail volume is unknown but should be on the order of a few million, many which users have forwarded to to other accounts so a small fraction is stored locally.

We also have few TB of web data that is hosted from a server in the same location that I thought to unify into the NFS setup.

Thank you for reading and your insight.