Play Belgium first, because you won't have to manage a large nation. Just make sure you build up the strongest army you possibly can and maintain it, because the Dutch tend to call on you quickly and often. I suggest forming an alliance with Britain (and possibly France) on day one so when you auto-GP about a year into the game they can help you because the Dutch will invade you to reform the United Kingdom of the Netherlands. It's completely winnable on your own, but signing an early alliance with France also makes them less likely to nom on you.