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You've never worked with air-tight vinyl, have you?

Here's the link for D&L: http://stores.ebay.com/D-n-L-Toys

The price on that used ball (your first link) is a steal. Go for it—I'll explain more in a minute. The soccer ball is closer to the typical prices. 

I recommend against trying to modify one of these beach balls because you'd probably never get it air-tight again. Vinyl is much heavier than latex and really doesn't stretch much. It will stretch a tiny bit, but is otherwise known for holding its shape. Vinyl will also tolerate much greater abuse. Since you already have a 3-foot-when-inflated ball, you know that it will support the weight of an average person, and the balls get tougher as they get bigger—slightly thicker gauge vinyl, extra heavy welds on the seams, etc.

I'm not sure if vinyl can be glued; most manufactured items are electronically welded. Some especially ambitious cosplayers will create closely-tailored costumes out of vinyl, which is risky since the material doesn't stretch much worth mentioning. They can get away with sewing vinyl (which I've heard is a real PITA) because they're not making inflatables. 

Why that first-linked beach ball is a steal: Violet's "spherical" blueberry costume in the 1971 WONKA movie was made from two hemispheres of styrofoam—like the stuff cheap picnic coolers are made from. A cavity was scooped out to hold actress Denise Nickerson, and the parts were held together by the outer costume. It reportedly took an hour or so to get into and out of the costume. Being kidnapped and rolled up in a carpet sounds like more fun, but at least the "hard" styrofoam rolled cleanly on camera.

Since you want a visible, transformative inflation on camera, perhaps you could do the same thing, only use two giant beach balls instead of hard foam. One in front, one in back. The vinyl will not morph as easily to fill spaces as a latex suit, however the vinyl beach balls would probably support you, as well as stand up to a great deal of abuse. For example, if pumped up into an exaggerated human form, you could "trip" and land safely with a nice, pneumatic bounce. 

A spherical shape, like Violet Beauregarde, would probably not roll neatly on inflatables without some mushing. Also, you'd need a spherical outer costume (made of denim, or some other durable cloth) to hold a spherical shape. Inflating the beach balls in something stetchable, like spandex, would allow the inflatables too much freedom. You'd look like a mutant pumpkin.

Still, no one said you have to go for spherical. Beach balls might fit down your legs and fill the body of your costume, but you'd need other inflatables for your sleeves. You might consult with Sievert, who has some excellent multi-inflatable images in his artist gallery. 

You might want help with complex, inflatable costumes. It's a lot like bondage, and it would be embarrassing if you got yourself trapped and the wrong person found you. Good luck.