An ST-E2 can be a gentler, more socially-acceptable, if expensive, AF-assist light. I bought mine for firing remote flashguns, but use it quite a bit for AF assistance in low light, when not using flash.
I just visited a dealer, and tried my familiar, much-favored 35L on a clean, pre-owned classic 5D. The resulting images were impressive! I had tried lesser lenses on this same camera earlier in the month, and was not overly impressed. What a difference good glass makes! I just may adopt this unloved 5D1!
Canon 7D2/7D/5D/40D/1D2N; Nikon F6/D700/FM3A/1Dx/Coolpix A; Canon 40mm 2.8 STM, 135L, 50L, 35L, 50mm 1.8 I, 100mm 2.8L Macro, 10-22mm EF-S, 400mm 5.6L; Nikkor 50mm 1.2 AI-S, 50mm 1.4G, 50mm 1.8D, 16mm 2.8D Fisheye, 180mm 2.8D, 100-300mm 5.6 AI-S, 18mm 2.8D, 45mm 2.8 AI-P, Micro 60/2.8G; Voigtlander 90mm f/3.5 SL II