If money is an issue, you really don't need it. You can plan a good itinerary without it. If you want to take midday breaks, just go back to the same park again later - no need to go to another park.
Park hopping is only essential if you have a short trip and you know you only want a 1/2 day in a particular park and don't want to waste the other half of the day i.e. you want to use that time in another park.
Park hopping can be nice to have, but it does put your overall ticket cost quite a bit higher - you have to decide if the benefits are worth that to you. With park hopping, if you enter a park and it quickly becomes a madhouse, you can hop to another. But if you do your research, or get Tour Guide Mike or use Touring Plans etc., you can figure out in advance which parks will be better on which days, so you might not need the hopping option. Also, if you're going at a slower time this probably won't be an issue.
Another hopping benefit is if you like to spend several evenings at Epcot (in the WS, for example) but you like your mornings somewhere else.
Bottom line: there are benefits, but it comes at a cost. It is usually not absolutely necessary if you plan accordingly, but it's a nice perk if you don't mind the extra cost.
You can add it at any time during your trip, so if you're not sure, try planning without it, and add it if you need to when you're there.