The direct fit R2 style halogen bulbs are probably your best bet if you retain the existing light assemblies. The design of the reflector and lens was made for the R2 bulb, so it will have the correct shape and location for the light source. Other bulbs like a H4 will not be exactly the same and may not provide a great light pattern. On the other hand, if you want to go to H4 bulbs (which offer LOTS of choices) then I suggest replacing the whole assembly (lens, reflector, etc) made for H4's. They are inexpensive these days. But you may want to check your local laws about swapping lights - some areas are strict and result in fines.
Regardless of which light you choose, adding repays will help. If you buy a premade relay kit make sure it has decent (heavy) gauge wires - particularly for the power source into the relay and from the relay to the light. I've noticed several of those kits come with too small of wires (in my opinion), which defeats the whole purpose of it.