Thoughts and opinions... I prefer to keep it simple. With careful cable routing I see no need for the added relay. I just ran heavy gauge cable ("0" I believe) from the starter to the back. 6 gauge from the starter to fender relay to feed the rest of the car. With the disconnect, you need to run the alternator lead to the back also, battery side of the disconnect. 6 gauge should do it with 125-150 amp breaker or fuse. If you are running lots of electrical load - accessories, big fan, ect 4 gauge for the charge wire would help reduce voltage drop.

I routed my cable under the car on the drivers side - similar to factory fuel line routing on the pass side. I grounded to the trunk floor through a brace under the floor to get more meat... then later added a 1 gauge ground wire from the battery to the engine block. Front chassis ground ran from the left fender apron just above the rail so I caught the heavier lip of the rail.

Starting current can be quite heavy... up to 300-400 amps? I don't think that breaker in the start circuit would take it and don't see the need for a breaker. A 200 amp switch can probably take it for the short time needed to get started.

I got the heavy gauge wire and terminals from NAPA.

Good luck!