A dress would be no use for easy access to certain clothes filling. I'd suggest a standard stretchy lycra top and little jersey stretch mini. Most importantly, you'll need to wear 2 or 3 pairs of tights. That means you don't necessarily need to wear any panties underneath whilst still retaining modesty. The point of that is that you get all the stretchy lycra against your skin (which feels lovely) and, most significantly, you can pour gloop inbetween the layers of the tights and it 'stays where you want it' for maximum fun, sliding down and sitting between your legs - great for squeezing and squelching around in there!

Don't forget to try some sitting in stuff too - paper plates loaded with slop and an old stool are all you need there - lift the skirt up from behind and lower yourself on to it so it squeezes up between your thighs. (Best to do this early on or you won't see the contrast with the clean bits well enough.) You can also have someone pull your tights open at the back, drop a plate of gloop in then let the tights snap back, trapping it in there.
For what to use, you want stuff that is heavy and sloppy but smooth and creamy. Best not to use anything with real dairy products as you'll end up stinking. If you're not bothered about food and don't mind doing some mixing beforehand, I'd recommend using modelling clay (available from your local artists' supplier) or directly by mail order (
http://www.potclays.co.uk/OrderProduct_ ... d=44109315) You'll need to break it into lumps, soak it in water overnight then use a drill-based paint stirrer to whip it up to the consistency of thick emulsion paint. (You could do it by hand - I have in the past - though it's quite laborious.) The coverage is amazing (just like paint), the smoothness/creaminess/feel is fantastic and it just rinses out of clothes and off bodies without a trace and no smell. Just make sure you water it down very well before letting it go down the plughole and keep water running for ages afterwards - you don't want it to set in the drains! (I actually try to retain as much in the bucket for re-use as I can, minimising the amount that goes down.) The only disadvantage is that you can't have someone lick you clean like you can with food!

I'll look forward to seeing the results!