I recommend the following:
1.) Stop by any hardware store and pick up a small toolkit that Black & Decker makes, they sell for ~$8 USD and have multiple bits for phillips head, flat head, hex, and so on. It comes in a sturdy orange plastic case that holds up extremely well. Every time I start a new contract this is the first toolkit I buy to keep at work. In addition, pick up: one heavy duty phillips head screwdriver and one heavy duty flathead screwdriver.
2.) Label printer. Preferably a hand-held one that uses regular AA or AAA batteries. Keep a spare set of batteries on hand as well as a spare tape cartridge. Label EVERYTHING at BOTH ENDS. It helps to also label each server with the network ID, model and serial number for those quarterly/bi/annual inventory audits when you're asked to verify what's actually in the server room.
3.) Small shears (heavy steel scissors) for cutting zipties off large cable bundles and opening god-damned plastic packaging.
4.) Pliers & needle-nose pliers, quite often things get bent that need un-bending.
5.) Cable tester. You'll want a reliable one that uses separate sender and receiver modules so you can leave the cable in place for testing.
6.) Smartphone. Install the best flashlight app available and keep the flashlight, camera and notepad apps on the 1st app screen.
7.) A server jack/lift if you can get the company to pay for it. Some servers can be a ***** to lift into place and sometimes it's hard to find someone to help you get something racked.
8.) Plastic bins that are stackable and have openings on the front that are easy to reach into. You'll want to keep a good supply of different length cables on hand and these will make it easier to keep them organized. Might also want to consider a large tackle box for fibre optics components and rack mount screws/retainers. If you get a large enough one, you can store all of the tools in it as well.
9.) A large flat crowbar (pry bar), you'll rarely use it, but damned if it doesn't come in handy when you need it. Might want to pair this with a large rubber mallet or sledge hammer
10.) Crimper, really not that important these days as most places use pre-made cables. In the last 7 years i've only needed to crimp 1 cable. Your situation may be different.
11.) Cordless screwdriver with a spare battery fully charged at all times. Last place I was at we kept 2 spare batteries fully charged and often went through them. These come in really handy when you need to swap out a dozen or more servers and have to undo a lot of screws or assemble/disassemble the racks themselves.
12.) Ear protection, either the big over the ear kind or just a box of disposable ear plugs.