Howard Miller

https://github.com/moodle/moodle

At the time of writing, the 'main' branch is the development version of 4.4. But with 4.3 only just out, this branch will be exactly that, a development version. So expect trouble.

That isn't a plugin it's a category of plugins.

If you want to know how to write one, https://docs.moodle.org/dev/Filters

If you want a filter that can be configured to do lots of different things, try https://moodle.org/plugins/filter_generico

Yes - Moodle performance is very reliant on its cache system. By default, data is cached to a directory in the file store. This isn't great at the best of times but will more than likely cause performance problems if you use a shared files area (e.g. NFS).

Moodle can (and should) be configured to use a dedicated cache server. We generally recommend Redis. See Caching

If by 'content' you mean the 'moodledata' or files area, then absolutely yes. Add a cache server to your list, though...
- web server
- database server
- file server
- cache server (Redis)

It's not always a good idea, though. You may be adding more complication for no benefit. As always... it depends. 

To be fair, although Moodle has been my day job and main income for longer than I care to remember, there are still many parts that make me want to bang my head on a solid object.

So, maybe not that defensive