Installing Lights from Scratch on electric 87 Marathon

[oldgrape]

I plan on installing LED Head and tail lights on my electric 87 Marathon (no lights at all installed). The lights are for a 12v system. I have the voltage reducer also. The tail lights are combo tail/brake/turn lights. I only plan to hook up the tail and brake function. I have no wiring harness. All I can find on line only discusses the use of a wiring harness. Trying to keep this as simple as possible. I have read if I get the new key switch (3 position) I can run the power through it over to the light switch so they only work with the cart on. I have seen several harnesses on line that have relays in them. Is the relay required? I now I will need to put in an inline 10 or 15 amp fuse. So I am no mechanic or electrical buff. Can some one advise me on my many questions?
1. Will the key switch idea work for me on my Marathon?
2. What gage wire should I use?
3. Where would I put the inline fuse at?
4. What do I need to add to make the brake light function work? (micro switch, pad)
5. If I install a radio later can I connect it to the key switch also?