Not sure how I ended up in this thread but I work on Ag stuff all day at our farm. Its a bit different than at a dealer though because dealers supply a lot of the tools for the shop.
I saw big tools and special crimpers listed above. Most shops supply tools requiring 3/4 drive and larger. Unless you find its something you are using often and its more handy to have for yourself, stick your money elseware. At least wait until you find a job and can see what they shop supplies and what they don't supply.
Most also supply everything for any type of crimping for various connectors. You will want some nice wiring tools for basic crimp connectors but the ones requiring their own dies like Duetsch, metri pak, amp etc the shop often supplies. Same with any special crimps for fluid hoses and such.
The Fluke 88V is a great meter, I just got one a little while ago. Also got loadpro leads and Dans book. I like the way his book is written, helped get me over the hump and into understanding how everything works and how its all connected. Dan absolutely hates the Power Probe III but I as well as many others find its another great tool to have even if it does have a point on it. Dan claims it can only find 20% of issues. In reality both tools are aimed at different tasks, his loads the wire to look for a voltage drop indicating corrosion or bad connection. The power probe supplies power or ground to a circuit which the loadpro leads cannot do. The power probe cannot detect a poor wire as well though because it doesn't load the circuit. The power probe is great for checking operation of things because you can quickly and easily supply power or ground to them. I would get all three tools. The other tools I like for wiring are pistol grip automatic wire strippers. Many companies make them. Snap on something 29 something 9" crimping pliers, its up to you but I reccommend grinding the wire cutter off the end and just carrying a cutter around with you. You will find many instances where that cutter gets in the way as you are trying to crimp a wire, or you may not notice it and you'll cut other wires on accident. Getting rid of the cutter lets you get into tight spots. For a side cutter look at a high leverage Knipex or Twinforce Knipex. High leverage is cheaper and works just as good, you just want that German steel, cuts like butter and never wears out it seems. A wire dismantler is very handy for trailer wire and other multi conductors. Mine is a Knipex but many others like greenlee and such make them as well. Then you will just need a butane heat gun. With those tools you will actually enjoy fixing wires versus hating it with cheap tools.
For hand tools, ratchets are where you want to spend the money. Get a good 1/4" locking flex and 3/8" locking flex. Personally I like Matco 88s, the slim head is much nicer than the Snap-on but I like snap-on's flex lock better. IIRC my 1/4" is 6" and my 3/8" is 11or 13". You will need a 1/2" at some point but honestly I don't find I use mine much because most of that stuff is done with impacts and tightened with torque wrenches. Get a good 3/8 flex stubby too, very handy to have. Or some zyklops.
Sockets are sockets when you are starting out, just buy what you can afford. The big craftsman socket bundle is always a decent deal.
Bit sockets are not created equal though, spend the money on quality bit sockets especially torx. The truck brands are all good, the German brands are great as well. You wont run into a ton of Torx on American equipment, but if you get into foreign stuff you will see a lot of Torx, Torx Plus and XZN. If you are strapped for cash up front just buy some nice bit sets from Wiha. Much cheaper than buying sockets. Not as handy to use but they will get you by.
I suggest not wasting any money on adjustable/crescent wrenches. Buy some Knipex pliers wrenches instead. You will love them for hydraulic lines and such. They work well too unlike a crescent wrench and the ratcheting action is handy for speeding stubborn lines off fast. While you are at it, buy some Knipex Cobras as well, you wont hate using adjustable pliers once you start using those. Great teeth and the self locking box joint is very nice. Other must haves from Knipex are a cobolt mini bolt cutter, I would suggest the 20* offset one with comfort grip handles. And as mentioned before get one of their side cutters. I like all of their pliers but the ones I listed are the ones they really excel above everyone else with. When you need some snap ring pliers look into their precision ones. They only do one direction and one size, may sound not as nice as one that can do multiple but once you start using snap ring pliers that do multiple things you will learn how much of a PITA those things are. It cost a lot to buy all dedicated Knipex ones, but I no longer cringe when I have to deal with a snap ring.
Lots of hammers and pry bars are helpful. Get an indexable pry bar too.
Radiator hose picks come in handy for a lot of things. I got the snap on set when it was a $99 special, love it.
Angle wrenches and crowsfeet will come in handy when working with lines in hard to reach places. Line wrenches are nice too but honestly if you buy good wrenches you wont have much use for a line wrench, at least I haven't ever had a time when I felt I wished I had some. Stubby ratcheting wrenches will come in handy too.
Theres really no answer to what you will need, like others said if you see a tool you probably will have a chance to use it on ag equipment. Some guys like having very few tools and making do with what they have, others like having every tool there is to offer and working with ease but with a bigger investment. Just start out with the basics and add what you find you need from then on.