There is no reason to spend the extra money on an impact socket set if you know you will only ever be working on machinery that needs manual sockets. However, impact wrenches must never be used with manual sockets.
You should either buy an impact socket set or just the specific impact sockets you require if you occasionally need to use a socket with an impact wrench.
A full set of impact sockets is essential if the majority of your work involves the use of an impact wrench. If your budget does not allow for both a regular set of sockets and a set of impact sockets, it is preferable to get the set of impact sockets because they can also be used with manual turning tools.
The answer to this question depends on the type of machinery you'll be using. Deep sockets are always useful to have in your socket set, unless you are certain that you won't need any.
There are often times smaller socket sets that have fewer deep sockets than standard ones. As a result, if you do buy a set with fewer deep sockets than standard sockets, make sure the deep sockets are available in the sizes you use the most frequently.
Once more, this will depend on what you plan to work on. While the average DIY enthusiast is unlikely to need Torx, spline, Robertson, or many of the security screwdriver bits and socket bits, they might be necessary for working on specific devices. It is worthwhile to look for a socket set that will include them if you are certain that you will be working on a product that does require them.
Even if it's only for tasks around the house, Philips, pozi-drive, slot, and hex (Allen key) screwdriver bits and socket bits are always helpful because they are utilized in such a wide variety of applications.
It is also possible to purchase screwdriver bit sets and socket bit sets separately. These are useful if you do not need regular hex sockets or wish to add to a socket set that does not contain any screwdriver bits or socket bits.