This is one of those "slippery slope" things. I'm currently doing a similar job on one of my trannys. Here is my take on it.
If you are removing the trans (which isn't that difficult and makes all work on it MUCH easier), then you can replace all of the seals. And while you are at it, you might as well open the 5th gear housing to see why 5th is causing you problems. And if you go that far, you might as well split the main case to clean and inspect everything (follow the resources on this forum). Even if there are no issues, you'll be shocked at how dirty the inside of a trans gets....even after changing the fluid. A ton of crud builds up in numerous pockets/cavities and continues to circulate after a fluid change [by the way, a typical "in car" trans fluid change does not remove ALL of the old lube and does little to remove much of the debris]. Opening the case will also allow you to replace the gaskets (another likely source of leaks) and makes replacing some of the seals much easier. And if you have things that far, then you might as well rebuild the whole thing <see the next paragraph>. Hince the "slippery slope".
However it gets REALLY expensive to properly rebuild the trans. And if there are no current issues with it, and a good cleaning/inspection does not reveal any further problems, then it isn't always necessary to replace every thing. That decision will partly depend on your lifestyle. Some don't have the time/willingness to go back in a couple years and work on it again if needed, and would rather do everything now. On the other hand others might not be able to afford a arbitrary total rebuild if it doesn't need it. Replacing all of the seals and gaskets isn't too expensive (depending on where you buy the parts), but things like syncros, sliders, hubs, gears, forks, bushings, bearings, etc, etc, IS expensive. And from the recent experiences of others, the replacement parts currently available are likely worse than the used components currently in your good working trans. Personally I don't believe in replacing parts just for the sake of replacing parts if they don't need replacement. And I'd rather not waste a lot of money on questionable quality parts. Plus I don't mind if I have to go back into it in a few years (assuming I still have the car). Which brings up another factor - what are the long term intentions with this vehicle and how much use/abuse do you give it? It is amazing how long a old trans will live with typical wear, so long as it isn't abused. So like I said, it depends on your circumstances and goals. But a good clean and reseal isn't difficult (my recent experience was the first time I've been into a Fiat tranny...but not my first trans). Your 5th gear issue might be a little more involved though. You'll find out more once you open the 5th housing.
