Is it crazy to not rely on a caching system like memcached nowadays ( for dynamic sites )?
Depending on the specific nature of the codebase and traffic patterns, you might not even need to re-write the whole site. Horribly inefficient code is not such a big deal if it can be bypassed via cache for 99.9% of page requests.
When choosing PHP or Python, make sure you figure out where you're going to host the site (or if you even get to make that call). Many of my clients are already set up on a webserver and Python is not an option. You should also make sure any databases/external programs you want to interface with are well-supported in PHP or Python.
I was just reviewing one of my client's applications which uses some old outdated php framework that doesn't rely on caching at all and is pretty much completely database dependent.
I figure I'll just rewrite it from scratch because it's really outdated and in this rewrite I want to implement a caching system. It'd be nice if I could get a few pointers if anyone has done this prior.

Rewrite will be done in either PHP or Python
Would be nice if I could profile before and after this implementation
I have my own server so I'm not restricted by shared hosting