Hello J,
It's good to see the instructions. From what I see the kit is perfect for electronic improvements. Now soldering is the best way to go here, we don't want anything to stop working after you close your model up, do we? "Twisted" connections have a tendency to do just that - stop working after they oxidize after some time.
For any action - cyclic on-off switching, blinking, changing intensity and stuff you'd need some electronic circuit to do it. While there seems to be plenty of space in the hull for such a device, you need at least some experience in electronics to install it. If you have such experience, go for it.
The good thing is you can use the base/stand as it is, without modification. All the mods will be in the hull. Polarity (+/- of the battery) is important here, you need to determine where the "plus" and where the "minus" is on the contacts inside the hull.
Now the values of the resistors depend heavily on several factors, most importantly on the type of LED you use. The newer LEDs make much more light per current unit than the older ones. To decrease the brightness you need to increase the resistor (more ohms) and vice versa. The good news is you can easily increase the resistor by "daisychaining" other resistor to it (it's called serial connection).
My idea would be to experiment - build a laboratory circuit to test the LEDs you have and experiment with their brightness. You see a LED you can see glowing in the darkness or even at a darker day is totally unnoticable on a bright sunny day, so it really would be good to check. Once you got the circuit nailed, you carry it over to the model.
I look forward to see your progress, so please keep us posted & have a nice day
Pawel