Welcome to NICO!
I have a 2003 M45 without navigation, driver memory or heated seats ... even without a sunroof! The fact is that the only thing I miss is the nav, so I ended up buying a Magellan unit that works very well indeed, and I take it on trips too, so that the rental companies can't gouge me. Plus, I have the European maps on it, so driving there is easy too.
Since I am the only driver of this car, the driver memory is not critical, and living in California allows me to forgo the heated seats easily. The only other person who drives my car is my son and he is pretty the same height as I ... so adjusting the mirror a tiny bit is about all I do after he drives it (our torso is a bit different). Everything else setting-wise kinda works for both of us!
Anyway, what I would suggest for you is to look at the full service manuals we have here at NICO (link is in my sig). These
may give you some indication of the level of effort involved in adding the driver memory or heated seats. My intuition says that the seats will be relatively easier compared to the memory ... but I don't really know.
Z