I've got a 2004 MT 8 with 96k miles on it so naturally there are a few issues at hand.
First and foremost my radio display is not working. In the beginning the display would become erratic and flash what look like something of alien symbols on the digital display if you can picture that. Now I've seen where the digital display and all the back lights for every button on the center console is out, however AC buttons and steering wheel radio controls still works. Looked this up and came to the conclusion it is more than likely bad mounting of the radio faceplate to the feed. This would not be the first time it has happened under my ownership. Before, the tech just removed the faceplate and put it back on properly after I had problems with the entire center console display flickering and then dying. So now I'm half tempted just to buy a used faceplate off of ebay with good mounts and put it on myself.

Secondly, synchro mesh in 4th has gotta be gone. I have to pretty much force/jerk the shifter into 4th to get it to stay when redlining. Is it as simple as buying new synchro and taking it to a transmission shop to install it or is there more to it that I am missing? If there is a lot more to it than that and it is costly, what are the repercussions of leaving the transmission as is with no synchro in 4th? I'm guessing just more wear and tear on the clutch, but then again I could be wrong.

Thirdly, my car is obviously getting up there. I got the car at 78k from my father who put pretty much just highway miles on it and I haven't done any differently since I've had it myself. I'm needing to get new ignition (BHR for sure), I'm expecting the clutch to start to go (no signs of it yet, but I'm expecting it soon), but my largest concern is the engine once I hit that 100k point (end of 100k engine warranty). I'm sort of debating whether I should get it rebuilt at 100k or if I should continue to run it until it starts to shit itself and get a new engine. I do push my car from time to time, maybe redline a few times on a day of a lot of driving and once on days of little in-town driving. Any input into the engine ordeal would be greatly appreciated. And if anyone who has experience with this car in the 100k+ range has any advice to offer or things to look out for in the near future, your 2 cents would be greatly appreciated.

Other than that at 96k, the car is in excellent shape all the way around and we'll see how long this toy will last

Any car will "shit" unless you take care of it. With the Rotary you have to keep up one the oil spark plugs wires and coils and your good. Other than that I would say your ok till the shitty tranys starts to go out on you lol

With all the money you are planning to spend on it, you are better off selling it and buying a newer one if you like it so much. Either drive it the way it is (+ tune ups) or get a new car.

^+1...sounds like a newer RX-8 is the way to go...can tell you like it by the way you are taking these issues in stride

Just have the engine compression tested to see where you are at, if its borderline you better make sure you drive it hard so that any engine power loss issues will show prior to the factory engine warranty ending. Also if it does show power loss prior to the warranty running out you will need to prove up oil changes.

The clutch wears per shift, not per mile. So if the car has mostly highway miles, your clutch could be in better shape than a city car with 30,000 miles.

As far as synchros, there was a time before synchromesh. Learn to double-clutch and rev-match and you may get a lot more miles out of that tranny, depending on what's really wrong with it. Or you could simply skip 4th; heck you've got 6 forward speeds. There was a time when 3 were standard :)

A transmission oil change may be in order, if for no other reason than the amount of metal particles that may be in the oil from the worn out #4 synchro.

I'm not that familiar with tranny rebuilds, though I did once help a friend fix a whine in his 911. We diagnosed it by driving around in various gears listening to the sounds; decided it had to be one of the thrust bearings, iirc. He got the part, we dropped the tranny, slipped in the new bearing, and it was good to go :)

My approach would be to just drive it until it gives up. Then maybe get the engine and tranny rebuilt at the same time.

Your engine could last a lot longer, if it's made it this long. Everything else you mentioned can be repaired or replaced outright. Be glad you don't have the AT. AT repair costs generally run much, much higher than MT. How much money are you willing to put into it to keep it maintained?

^ Yes I have heard of the Renesis lasting quite a ways above 100k if maintained the way it should be. If I do start to feel power loss though after 100k, I'll probably get it compression tested and if it's for sure the engine, get it rebuilt before permanent damage occurs.

I'll probably just run down the transmission until it shits itself and buy a used tranny in good condition. After reading replies that seems the way to go rather than paying parts + labor to get it fixed. I hardly run into situations where I'm grinding 4th anyways...because lets face it, how many times do we redline shift from 3rd to 4th in everyday driving? I'm not the type of driver that redlines everywhere to be some sort of 'rice burner', the 8 has way too much class for that. And as for the clutch, just wait for that I suppose and get a good quality replacement when time comes. I have had absolutely no problems with it yet since I do travel on the highway so much and I'm very good to the clutch when I am in the city.

I'm still sort of iffy on just buying a new 8, I'll look into it as an option once I get back from deployment next year. I'll just have to cross my fingers nothing catastrophic happens to the car before I decide to sell it for a newer model

Yes, I agree with Nubo, that is to replace the Transmission and Diff fluids, how long ago where they done?

Replacing 4th gear Synchro is quite involved and many hours of labor..

To check your clutch plate, with a hot car, while in 4th gear, rev engine to 5500 revs hold it there and let the clutch go quickly, if your engine dies or stalls, your clutch is OK, if it slips a lot, your clutch is on the way out or is already stuffed.

Have you prayed to Rah lately? Our beloved Sun God.