I am by no means a professional but I have worked on conversions and with green stuff for quite some time and through research. I think I may be able to give you a little insight.
First You need to have an idea of what you would like the model to look like in the end which would help justify the tools needed.
Things to use:
1. Bits. They are great because well... they add flavour to models and with all the amazing bits out there you can truly make a unique model.
2. Thin Copper Wire. To my understanding you are making this from scratch so building a skeleton may not be a bad choice.
3. Mighty Putty / Green Stuff. I mention both of these because Mighty Putty is stronger but harder to work with as far as sculpting and cheaper. So for bulking up the model perhaps that may be a good choice. Then for the more detailed parts thats where the GS would come in.
4. Sculpting tools. These are important, You can pick up some great ones from any hobby supply store.
Hope that may help a tad!
Best of luck!
Chaosftw