Research strategies
You have to decide what your longterm strategy/style of play is. If you are going to take a peaceful style of play, then you want to go like gangbusters on infrastructure to build tax collectors, judges, governors, and factories. The earlier you get these infrastructure levels and the earlier you make the upgrades, the more they pay off.
Beware however that infrastructure requires a beaucoup grande size portion of your budget, so everything else gets sacrificed.
If quick war and expansion is your style, then heavy investing in land tech is a must.
For either strategy, you cannot afford to get behind in trade. You must keep on par with your neighbors or risk getting kicked out of your own center(s) of trade, never mind in other CofT outside your territory. Being able to do this to another player (monopoly in his captial CofT) is great fun, btw, and with some diplomatic expenditures, AI players will not do a DofW in retaliation.
My preference is to stay on par with naval and one level up on land.