I have a rule where I only allow myself to use presets for analog instrument (pianos, organs, guitars). I use nexus for that. For everything else, I do it from scratch. I just feel better with myself when I have something done that sounds good and I know I sound designed it by myself.

Layering nexus's synths can also be a good idea for some, but I wouldn't advise anyone to use a nexus preset as a main sound because those synths are everywhere and they can make your production sound cheap. 

Guitar mainly. I think producers should seriously execute an instrument before they start thinking about doing electronic music. It's like someone trying to manage a full orchestra without they having an instrument of their own.

I normally don't think about a key, but just a chord. Then I see where my creativity takes me. Must of the time I end up in the same key as my chord, but it's not always the case.

For example, If I'm in playing a C major chord sometimes I stay in the key of C, but I might change to the key of G if I want to go that direction. Either way that chord would be coherent to both key signatures.

Then you can build different parts of the track and modulate between key signatures.

There's a lot of ways you can do it. If you don't have a lot of experience with music theory and have a keyboard in front of you, you can just always write in C (all the white keys) and then transpose if you need to. Some of my fav producers do it this way.   

Well, you don't need hardware at all, but if you have a lot of money to spend in high end analog pieces of equipment... go for it.