Is there an exotic hardwood dealer in your area? It's always waaaaay better to examine the stuff in person before buying. There are two main types on the market (well, actually there are more but for the purposes of arguing about ebony there are two commonly available) - Gabon ebony and Madagascar ebony. The Gabon stuff is generally more purely black, Madagascar is more brownish and usually has streaks of brown in it. The fewer brown streaks the more desirable (and expensive).
Ebony is prone to cracking in the curing process. Look closely for cracks. Also cracks can appear when the wood is opened up. Not the end of the world - wash thoroughly with acetone and trickle/squeegee epoxy into the crack. Trick: create a pile of sanding dust from the ebony and use it to dye the epoxy black, and the crack will completely disappear. I recommend the same coloring trick with the epoxy you'll use to glue it onto the fore end. The glue line will totally disappear. (And you should use epoxy, IMO, after washing the oily-ness off the joint surfaces with acetone. Yellow glue doesn't bond to ebony as well as epoxy does in my experience, and don't get me started about cyanoacrylate or Gorilla Glue.)
Use as big wood dowels to anchor it onto the stock as you can get away with. Bigger = better structural integrity. Another technique is to mill/rout a slot in the barrel channel that subtends the walnut and the ebony both, then epoxy a floating tenon in place. An end grain-to end grain glue joint is the worst joint and you need to do everything in your power to effect a mechanical lock.
