Hi,
I absolutely second good lighting in the garage. I have seven small 150W floodlamps at ceiling level around my generously sized single garage - which also cover all my personal heating needs on all but the coldest winter evenings!
As regards tool organisation, I now have three multi-drawer cabinets and by regularly putting my tools away, the searching time has been vastly reduced!
Whilst building my Eleven, I always used to lay out the parts for each stage, read the build guide, sleep on it then assembly / installation the next day always went well (or a subsequent day after I had worked out a better method after thinking things over more).
As regards things I couldn't do without:
Plenty of bench space - minimum enough to comfortably sit a rigid axle on (or the biggest single component / assembly of your car)
Air powered riveter - no more "nips" as the hand gun snaps shut.
Solder station (posh soldering iron), Heat Shrink Tubing and Car Builder Solutions' good terminal crimps (that put the "double fold" into the connectors where they grip the cables.
1/4" socket set and extension bars - small and easy to fit into spaces and used for most assembly jobs unless big nuts/bolts are encountered and high torques needed.
A good battery drill / driver (and sharp drill bits and step drills in a range of sizes).
Good quality sidecutters - mine are £30 insulated CK ones that I have had for nearly 30 years now. My plumbers shears are nearer 40 years old - and I was given them by a steel fixer on a site so I have no idea how old they really are!
Cable tie installation tool - that tensions and neatly cuts off the tail (another use for the sidecutters). I have a scar on my arm where my Sunbeam Lotus got me with a sharp cable tie end that had been fitted and cut off by someone else, so I get properly OCD now ensuring that all ends are trimmed square and flush.
Aluminium racing trolley jack - low entry and light weight
A good stock of fixings such as nuts / bolts, P-Clips, Cable Ties, Saddles, Electrical Connectors and Covers
DAB radio in the corner of the garage - particularly Sunday afternoons for Johnny Walker's "Sounds of the Seventies"!
Simon