So, didn't realize this thread was still going but I really appreciate all the advice. I'm not a car guru by any means, but I'm leaps and bounds ahead of just about everyone I associate with so it's nice to have some place to go when I have questions.
To answer a few questions-
Why am I even thinking about selling the car- The Impala has been taken care of, at least as far as I can tell. No leaks, locally owned (bought when new in the same town I'm in) and had plenty of service records (I got the carfax before I bought it). The biggest reason I'm even contemplating selling it is pretty simple- I am unfamiliar with this iteration of the Impala and their reliability. Some people say they are solid, but a majority of people I've heard from say to run from these cars as fast as you can. I also never really feel solid about the car even after I repair something or perform preventative maintenance. Coupled with that feeling, there are some issues with it such as the infamous 4T65E hard shift (although the shift kit seems to have really helped- hasn't shifted hard yet), it's in dire need of shocks/struts and intermittent hard starts (what I would call stumbling starts). With that said, I have put a lot of work into it (see first post) and it's hard to give up a car that you know has had that much work done to it. Also, I hope that the intake manifold gasket fix was in fact done in 2003. That would certainly make me feel better about it.
Why replace it with an older car, and a Lexus no less- I'm not really worried about age as there's not a huge difference to me between the two. Now, why the Lexus. I'm not interested in the badging as much as I am the powertrain. If this was a Camry, I'd be just as interested. Unfortunately, decently cared for cars are hard to come by around here it seems (granted, I just check Craigslist passively from time to time). These engines/transmissions just have a much better track record in terms of reliability and longevity.
Now, I've given it some thought and I've decided to just stay put. I'm going to put away money for a future replacement car and drive this thing until I have to tow it to the junk yard. I've never retired a car in that fashion, but there's a first for everything. I'm hoping that I can get quite a bit more out of this car as it is in really nice shape, but I guess we will see.
Thanks again for the input. It was an interesting read and a lot of solid advice!