... I see most explorer builds don't have cargo racks. Is there a use for one? ...
There are two practical uses for cargo racks for exploration.
1. Adjusting fuel use per jump. When you have a cargo rack, you can add weight to the course plotter which reduces the plotted range per jump, which saves fuel.
2. Temporarily holding synthesized limpets.
... I will have repair limpets and hestsinks. ... Any tips for a new guy? ...
IMO the Diamondback does not have room for repair limpets.
To use a repair limpet controller you have to also have cargo space which means it takes two slots
Here's a sample build
https://eddp.co/u/UFuP9mC7
You have three optional slots.
You can do without the AFMU if you do not plan to supercharge from Neutron Stars. Otherwise it is absolutely required.
You can do without the Shields if you are a very careful pilot or you never plan to land on planets.
You can do without the SRV if you never plan to land on any planets.
You can sacrifice shields and SRV for repair limpets if you want to but I wouldn't recommend it.
Landing on planets and exploring the surface is a part of exploration for many.
You will also need to land and use an SRV to gather materials for jumponium, or AFM refills or other synthesis.
When you are out in deep space, damage to your ship is rare and unlikely unless you just don't pay attention when you're flying.
1. Damage can come from impacts with a planet or asteroid. This can result in hull damage if the impact is hard enough to take down your shields.
If you are
bad at landings, shields are highly recommended.
If you are not bringing an SRV, there's no reason to land anyway.
2. Damage can come from heat. When fuel scooping or when you jump into a system too close to a star.
Heat won't damage your hull until it reaches 150% which is unlikely in either scenario. Heat will not damage your power plant at all.
All other heat damage can be repaired by an AFMU. So many consider a heat sink launcher unnecessary.
3. Damage can come from Emergency stops. Hitting the exclusion zone of a star or planet too fast will cause an emergency drop from super cruise and will damage EVERYTHING including hull and power plant.
This is the only thing that can damage your power plant. An emergency stop can also be caused by attempting to repair your thrusters or FSD while you are still in super cruise.
4. Damage will also happen when supercharging from a Neutron Star or White Dwarf. Normally this will only be damage to the FSD which you will need to repair using the AFMU before the damage gets below 85%.
Supercharging from White Dwarfs is NOT recommended. A WD only provides a minimal boost and the exclusion zone on a WD is so large that the chance of hitting it is high. It's just not worth the risk.
If you hit the exclusion zone on a WD or NS and you are dropped out of super cruse while you are inside the jet cone, your ship will take a lot of damage to all systems.
See this video for a demo of how to supercharge and how to use the AFM for repairs at the end.
https://youtu.be/8C0YMEI7guc