You can get untold of miles out of a 1974 Airhead. When it breaks then you can say you have been there. Nobody can say when that will happen or for that matter that it absolutely has to happen. It's just a matter of it has happened to many of us. BMW changed a lot of stuff in many of the years of Airhead production. The first edition 5 speed trans lasted one year, 1974. I guess it's the one really really bad thing we like to harp about.
When it comes to Airhead boxes there is one thing to know so you can swap them around. All boxes made from model years 1970 up till and including 1980 are long input shafts. This is the same shaft with the splines that we talk endlessly about lubing. Model years 1970 trough 1973 of course are 4 speeds but you should maybe know that you can use a four speed /5 trans on a bike that originally had a five speed. As long as it's a long input application. Long input boxes use the flywheel and then in 1981 we get the short input transmissions that use the clutch carrier.
So you have a 1974 bike and are maybe worried about this box being a problem some day? If a 4 speed drops in your lap grab it but I recommend you try to get a 5 speed. The 4 speeds are good trannies but I think the 5 speeds are better that's all. I will get no end of gruff from riders that want me to tell you that a 4 speed is OK. Alright, it's OK. But a 5 speed is what you want.
You are therefor looking for a 5 speed for models 1975 thru 1980. Any Airhead box will do including the early R65es or an R60.
If you are feeling really adventurous and wanted a better clutch with faster acceleration then you could find the parts for a clutch carrier and use a short input shaft trans. But if you are thinking of going to all that trouble it might be better to just get a later model bike to start with. Don't worry it has been done numerous times. And since you are starting with a 1974 bike the swap to a clutch carrier is the most involved, it is easier after 1976.
So you want a newer trans. that's all. If the one you have is working don't worry about it. A good deal can still be had on these boxes if you are patient.
I advise you to follow any and all threads here about transmissions. There are some finer points we cover and the subject of riders rebuilding their boxes is never ending.
I've rebuilt one box myself but I'm using a used box right now. I need to rebuild another.
BTW, your assessment in the last post was right on. The 1974 box is a weak point. But there are certainly other things that can give you heck while you're worrying about the trans.
BTW, if you can come with a suitable replacement and the '74 box still works it can bring decent money because so many riders think there's some advantage to owning a kick starter trans. Or it makes a good spare maybe.