RE: What gun should I buy
He probably gave you god advice not to shoot a rifle with a badly cracked stock, but it also sounds to me like he's more interested in selling you a new rifle than fixing the problem at hand. Liquidorange gave you good advice on where to start. I'd call Mossberg first. While the 800A may not have been made for 15 years, they might just have a spare stock or two sitting around somewhere that they might sell you for much less than a new rifle. If that doesn't pan out, most of the stockmakers out there will custom inlet a stock for you for less that a new rifle as well (it IS your grandpa's rifle that he handed down, after all). As long as you don't want to stick AAA Exhibition Grade French walnut furniture on it (absolutely beautiful, but VERY pricey!), it should be a good value.
Good luck,
Mike