It sounds like you may have the same vintage MBP as I do (Core 2 Duo). I agree with your attitude about the OS X "updates" but find that Snow Leopard
(10.6.8) is my sweet spot, as it is for a lot of others. Lion and Mountain Lion are not my cup of tea either. The problem is that Apple did a lot of strange things to BSD Unix when it first made OS X, and then kept undoing their mistakes
with each release so that by Snow Leopard it was mostly reasonable. Then they got serious about messing with the GUI, which is where I get off the train... If your MBP can run Snow Leopard I would recommend you try it, if only on an external drive. Pretty much all current
runs on Snow Leopard. Just like XP in the Windows world, so many folks are either unable or unwilling to "upgrade" that Apple can't pull the plug
I have found that Airmail and other Windows programs work just fine on Linux
with WINE - no need to keep Windows around. Worst case, dual boot Windows/Linux. I am a fan of Linux Mint - try it on an external box, or use a virtual machine with (free) VirtualBox. And of course OCPN works on all of the above - thanks Dave!
As for a 10.4 build, I have serious doubts about a PPC version, but it might be possible for x86 code. I'll try to build for 10.6.8 first, and if I can do that I will work back to 10.4. I do have the required resources to try it. OTOH you might relent as far as Snow Leopard...