water based clays can also cause problems with inhibition as some rubbers will not cure against wet surfaces; or if they do they may not produce the result you are looking for...
which ever rubber you use, you MUST know about it's compatibility with the model surface or what the model is composed of... otherwise you risk destroying your model and wasting money on mold making material because the whole experience failed.
if it's something you don't want to devote time to, hire an experienced mold maker; here's a list of mold makers for hire:
www.smooth-on.com/p101/Moldmakers-Exchange/pages