I started with this (no longer sold):
https://www.amazon.com/gp/product/B002VXN6TG/
I don't use most of the SAE sockets, but if you get a metric-only set look for one that'll still cover the largest and smallest fasteners that you might find on the vehicle. His car might also have Torx screws which he'll need the bits for.
The case fits nicely in the 3rd row compartment, which is lockable and nicely out of sight of passerby. Also in the 3rd row compartment, I have a briefcase ($5 from Goodwill) with stuff like:
-Vice Grips
-Channel lock pliers
-Needle nose pliers
-Wire crimping/cutting tool, wire and crimp connectors
-Philips screwdriver
-Extra big flat screwdriver (AKA prybar)
-Cheap digital multimeter
-Extra extensions for the socket wrenches
And the breakdown kit elsewhere in the back:
-HF breaker bar (AKA lug wrench) with appropriately sized socket for lug nuts. OEM wrench bent when I tried to use it.
-HF bottle jack
-Jack stands
-Gallon of coolant, quart of oil.
-Spare belts.
All bought piecemeal from HF and Amazon over the last 3 years, as I need it... There's more junk kicking around but that's the stuff I've actually used at the side of the road.
A more sensible person might store all this in the garage, but I like having all my tools with me on long trips.