Nope not all the same.... they vary in the number of gears, (some 4 speed, some 5 speed), gear ratios, output ratio, etc. etc. Most parts are interchangeable to a certain extent from one to the other however. i.e. they will all fit in the other cases, they should all fit on the other shafts, but gear ratios on individual gears may be something you want to take a look at. Count teeth on the gears and divide to see how close you are.
Any 700 series is a good candidate for build, some are simply better than others as a result of incorporating steel input and bevel gears from factory.