The Pioneer DEH-80PRS and Pioneer Stage 4 DEX-P99RS are pretty darn good head units. Lots of processing power, lots of features, and pretty sharp looking units. However, they will be significantly more difficult to navigate than your old Nakamichi head unit, especially while driving. You have to go through different menus to do almost anything, including basic equalization adjustments. For me, I want simplicity when I am driving. I want to keep my eyes on the road as much as possible. Thus, I am still using an Eclipse CD8051 dead head unit which is similar in operation and performance to your Nakamichi. But, it is old school and lacks most of the modern day features (bluetooth, USB, satellite radio, etc.) If you want to stay simple with your head unit, Alpine makes some quality head units (CDE-164BT, CDE-HD149BT, CDE-HD148BT to name a few}. Oh, by the way, it really also depends on what model year your Odyssey is. It seems like around 2006 or so that almost every car manufacturer has started making their cars that have everything integrated into the sound system, thus making an aftermarket head unit addition all but impossible. If you have an older model Odyssey, then you will probably be okay in finding an aftermarket head unit.
For speakers, if you stay passive, I'd look at Morel. I'm running the Morel Tempo Ultra 602 two-way components and absolutely love them. For 6.5" drivers, they have plenty of bass and the mids and highs are phenomenal. I can play music for hours at loud volumes and not get ear fatigue. But, there are other manufacturers that are very good, as well, such as Hertz, Focal, Hybrid Audio Technologies (HAT), Image Dynamics, DLS, etc.
If you want to go active, there are plenty of quality raw drivers at
partsexpress.com. Since I am not running raw drivers, I cannot help here but there are many people here who can give you plenty of info in that regard.
There are quite a few high-end amplifier manufacturers out there (Sinfoni, Mosconi, Audison, etc.). I am running Arc Audio and, again, love what I am running. I do, one day, want to upgrade to a Z series Zapco amp. Zapco has me sold on their high-end line. They are just more than I can afford, currently.