Others have really answered but here's my tuppence worth ;-)

1. Will MoI and Silo compliment each other

Yes - different beasts. You can start in MoI and then export and finish in Silo, though not the other way round.

2. Is it overkill to have both MoI and Silo

No - gives you options depending on what you want to model. Most people seem to collect apps as there is not really one that fits all needs :-)

3. If only one is needed to model what ever might be asked of me to model, which would you choose (non biased please)

If you only do mechanical type hard edged modeling then you could perhaps just use MoI. But i agree with what has been said before, polygon would be more flexible for different purposes, both hard edged and mechanical. But with the price of each, get both.

4. Would you say forget MoI and Silo and go for....

Only you can answer this one. I also came from a 2D graphic background and this depends on budget and needs. I've ended up with a number of applications. I've settled on modo as my main app, MoI for nurbs and zbrush for organics (a lot of learning going on just now). Along the way I've tried Vue, Poser, Carrara, Hexagon and Silo but none of them really 'fit' with me. Hexagon/Carrara combo came closest before DAZ took them over.