It sounds like you are proceeding with the Cateye Astrale, which is a very nice choice in the wired category, having both rear-wheel speed sense and cadense sense. The cateye "double wireless" unit mentioned in an above post is a nice wireless choice for a speed and cadence sensing unit. Cateye also makes another wireless model that does speed and heart rate, but not cadence.

I just found a nice deal on ebay for a VDO wireless unit with both speed and cadence - it also supports rear-wheel speed sensing for trainer use. Having both the speed and cadence sensing wireless adds a fair amount to the cost, versus one of the many speed-only wireless units (like the Cateye Micro Wireless).

Unless the product description specifically says rear-wheel sensing, you shouldn't assume that you can do that. In general, units with wireless cadence will also tend to support wireless rear-wheel sensing, since the transmitters for both the rear wheel and the crank are probably both mounted on the left-side chainstay, and both need sufficient range. However, there are some wireless units that are wireless for speed, but use a wired cadence sensor. These tend to be good for front-wheel speed sensing only.

Poring over the various features is a little confusing. Performance Bike's web site has a nice assortment of products listed, with feature lists, user reviews, etc. If you really want to dig, you can go to the various makers' web sites (Cateye, VDO, Sigma, Vetta, Polar, etc.)