first I would say - "joypad" or "mouse/keyboard" depends on the games you want to play. If you want to play side-scroller, jump & runs or beat´em ups - go for a joypad (if your game of choice supporting it at a PC). If you want to play shooter, RTS or adventures go for mouse/keyboard.
The newest hype are 50G sensors - my new Razer Viper Mini has a 35G sensor and it is quite fantastic especially for shooters. But all in all the Viper mini will only get 3 from 5 stars (my point of view).
You should have enough DPI, don´t know if anyone will need 8500 DPI (or up to 16000), but having up to 6000 should be good enough.
Why do I use the Viper "Mini"? Because of my small or short hands, it is important that the mouse will fit your hand. The buttons should be where your fingers are.
Another point on finding the right mouse is the weight, for example the "Viper Mini" is an extremly lightweight mouse with only 61g. A "Logitech G502 Hero" has more then 120g and you can add more weight to it, depending on your preference. At the moment lightweight mouses are hyped and my opinion is, why stressing your hand with 120g if you can have a mouse with half of the weight. After a short time you get used to it and I´m very happy with those 61g from the Viper Mini. But in the end, it is your personal choice.
In addition, buy a good mouse-pad, I personally use the Razer Firefly V2 - it is really important especially in shooters or RTS games. What kind of pad/mat (hard or soft) depends on your personal preference. As you can see with the Firefly V2, I prefer hard mats.
It is important to choose the right switches. Gamers normally prefer "mechanical switches" - the best "switches" are from "Cherry". Which kind of Cherry switches you should buy depends on your environment. The red switches are the best (most gamers will tell that) but they are very noisy. If you buy the "Cherry MX silent switches" using the keys is a lot more quiet.
I personally use the MX silent switches from Cherry in this keyboard: CORSAIR STRAFE RGB MK. 2
In the end, companies like Logitech, Razer, Corsair and a lot more creating a lot of Tools for Gamers, normally you will not be fooled to choose one of their products. If you will find a mouse, pad or keyboard which fits in your budget you can google for tests and then you need to decide if it will fit your needs or if you need more budget ;-)
What else to say - finding the tools fitting your needs best could be a long way. Start it with searching for some tools, read tests, decide to buy them, use them and then adapt your needs. Maybe another mouse-pad or mouse or keyboard will suit you better, but this can be estimated better if you have experience with some gaming products.
Definitely. If you're into games like World of Warcraft, then you may want one of the mice designed for such—their unique attribute is lots of buttons to which you can assign different actions, which speeds up your play. I had one with ~20 buttons some years ago—I don't play WoW etc, I was hoping to turn it into a sort of 'super macro' input for general Windows & Office work. But I didn't suit it, couldn't develop the muscle memory to pick out the right button reliably without looking.
Yes indeed. There are 3 ways people grip a mouse—claw, palm, finger. Make sure you know your preferred way and choose a mouse to suit. Very generally:
Claw—rare grip, smaller mouse.
Palm—bigger mouse, moved by arm.
Finger—smaller mouse, moved by finger, more precise with high DPI setting.
Another factor to consider is if the KB's programmable. I have an old Roccat Isku KB which has 25 programmable keys, and 5 different profiles. This means you could set up a profile for 5 different games you're playing, with a load of shortcut keys for each game—the driver already came with a bunch of preloaded profiles for the top games of the day back then.
I'm a casual gamer, so I don't use those functions for gaming, I use them for work-related stuff—a fine productivity boost once you get used to it. Mine doesn't have the mech keys, so I'm not recommending it for serious gaming—just consider programmability.
I am currently using the Corsair K70 keyboard and Razer's Deathadder Elite. The elite was on sale when i got it for less than 50 which, in reality is all i think most users need to pay for a good mouse. But the corsair keyboard is great as well, if you like the clicky clicky of mechanical keyboards.