I like the idea of starting off in soccer gear or rugby gear (long socks, shiny shorts, fitted t-shirt) and getting slopped, slowly stripping off to reveal swimming trunks / tight boxer shorts / compression shorts underneath.
I can't get enough of the feeling of filling nice tight underwear with slop until it's spilling out everywhere - it's such a great feeling!!
As for substances, brightly coloured natrosol / methylcellulose is hard to beat for visual impact on camera, but custard looks and feels great too.
The best / worst gunge I've ever used (by which I mean the most yucky / humiliating, which is good or bad depending on your view - for me it was the best!!) was a bucket I filled with a disgusting mixture of porridge, baked beans, mushy peas, golden syrup, chocolate syrup, strawberry syrup, pasta sauce and ready made jelly that I squished into big lumps. That was really something - sloppy, stinky, silky and stringy! Felt great inside my clothes, and the cleanup wasn't actually as bad as you think it might have been.
I agree with alsmartuk that the kit sounds great and let's have some speedos underneath and see how much natrosol you can fit in your speedos before it starts to spill out!