All the dryer does is absorb any moisture in the system, basically nothing but an empty can with dessicant in it. If your unsure about the one your using pop it in a 250 deg oven for about an hour to re-activate the dessicant.
Make sure none of the system has any mineral
oil from using R12 as R134 uses PAG
oil and they will gel if mixed.
Another important item is your expansion valve, this is located between the condenser and evaporator. (opposite side of system from the pump) It's primary purpose is to regulate "flow" of refrigerant through the system. The trick of course is knowing which size to use as this is the item you "tune" to your condenser / evaporator size.
Try to use as many stock components as you can as the system components do need to be somewhat balanced to each other.