Have a look here, Susan. DC Rainmaker does the most thorough reviews of GPS watches on the internet, for my money.
I started off for a long time with the 405 which is pretty similar but lacks a few of the 410's features. It's all you need unless you ever decide to step up the distance and need longer battery life, or do trail stuff where you need better navigation functions.
There's no easy answer, as every individual loses a different amount of salt in their sweat.
There's probably some clever way to work out how much salt you're losing per hour of running, but really the most practical way is to experiment and find what amount of a particular brand of tablet / powder / drink you can stomach during a race without upsetting your stomach but still staving off cramp.