If you get the Thixo agent you can spread it on with a spatula or other tool. This is a really good idea if you have a lot of undercuts as you can place the silicone where you want it to any thickness.
condensation cure does not suffer from cure inhibition, that can be caused if you use the wrong clay or wear rubber gloves while handling addition cure silicones.
There are tutorials all over the place for making moulds for this type of project, there are several approaches to it. It's going to be a case of what you think you will be comfortable with on your first outing.
when you come to casting you basically want to create a skin, by sloshing or painting the resin in the mould. when that is done you can use a filler additive to the resin to fill up the void. This saves money and cuts down on heat produced by such a large mass of resin. Or leave it hollow...
Here is one approach shown by Smooth-on, obviously showing their products :
https://www.youtube.com/watch?v=sOyBawt1-Ecif you like the look of the Smooth-On approach , you can buy their products in the UK from this company
http://www.benam.co.uk/They are very helpful and approachable guys at benam, they went the extra mile to help me out once, so I can't praise them enough.
I think you can see that it will not be simple or cheap to do want you want to do, but it is worth it
Good Luck!