Thanks for all the feedback!
Finding the $20K - $24K for a newer '07 323i or even 328, and possibly a Certified Series, isn't out of the question, but obviously at some point it becomes a case of diminishing returns.
Do you think the potential savings from a newer vehicle with a CS warrantee might come close to justifying the extra price? Or at that point would I be potentially spending a lot extra for not much gain? With that $5K - $10K price difference, it seems like I could afford a lot of repairs and still come out ahead in that sense.
And when the E90 did eventually start needing the repairs that all cars need sooner or later, would those tend to be pricier (and/or more numerous!) than what an E46 or E36 would need?