you absolutley need a 0-30 low pressure gauge. If you use a high pressure gauge 125-150psi , the degree of accuracy on a high pressure gauge could vary as much a 1-2lb which would be critical to setting tire pressures.
I liked my tire pressures at 6.5psi with the altered, sub 6psi I could feel the car swaying in the sidewalls mid track, over 7.5psi it would bounce in the shut down on rough tracks. Moroso do a fairly cheap 0-30 psi gauge for about $50 if I recall.
Other trailer / cart tools I carried, foot pump incase we got held in the lanes too long and pressures dropped, spare tire valves, dzus tool and fixings, fuses, a short length of wire with piggyback terminals incase we needed to hotwire a failed circuit, AN wrenchs and rags for leaks, chute cord for packing the chute, dial in marker, zip ties for quiks fixes and slave battery and leads. Would generally have a smaller selection of tools in the towcar too.