I've inherited 200K lines of spaghetti code — what now?

Image and archive the extant system. In triplicate, including off-site. (Don't assume anything you're told about existing backups is correct / correctly and safely executed.)

Select and teach them version control. You want to be able to document and roll back changes made from this point forward. Mantra: "First, do no (irreversible) harm."

This code base will contain knowledge/configuration from people who are no longer accessible. Even the current experts might have to expend considerable time and effort in trying to recreate some of it and/or and understanding of those parts specifically.

So, you aren't just going to rewrite it. A first step, aside from ongoing work, would be to document it.

Some of the people you are working for are going to want to get on with things. A primary focus in your education might/should be to make them read some of the better documented cases of such systems "blowing up" -- 'the bug that ate Rochester', and the like. Convince them that now is the time for some risk management, while their luck (hopefully) holds.

Once you/they know what you have, some of those "best practices" may make more sense and sink in better when they can be applied to something that they actually, specifically, and directly care about.

Did I mention, version control?

Oh, and establish from the start that you are the expert in this, and that this is your job. Some egos are likely to try to push you around, unless and until you push back sufficiently well.

Framework sauce...maybe some MVC-balls?