This is my garage from a few years ago before the wall behind the Mustang got taken over by kids' toys and athletic equipment, but I'm going to attempt to give a concise list of what should be in the garage of the used car buyer.
A large rolling tool box, like this Best Choice Products rolling tool chest, offered for $114.95 on Amazon, but you can get Craftsman version from Sears or the Husky version from Home Depot.
Large Craftsman (or equivalent) 300+ piece mechanics tool set $219 from Amazon.
More combo (box and open end) wrenches to fill out the gaps in the wrench set above like this 24 piece Craftsman set offered for $69 on Amazon.
Set of ratcheting wrenches like this 20 piece Craftsman set for $70 on Amazon. I use these little guys all the time, it is like the ultimate fusion of a wrench and a socket set.
Socket extensions and universal joint swivel pieces -- you can piecemeal these from Harbor Freight and garage sales or buy a big kit like this 20 piece Craftsman set for $40 on Amazon.
A large set of assorted quality screwdrivers like this 17 piece Craftsman set offered for $27 on Amazon. You can't just do with a few random screwdrivers, you need the stubby handle versions, the small head, large head -- trust me, you can never have too many different screw drivers to chose from when you really need a chisel.
Pliers are needed in all sorts of situations, get em cheap and toss when they yield or rust...like this Stanley 3-piece set for $13 on Amazon....or this Workpro 7-piece set for $13 on Amazon.
The above pliers kits usually include some diagonal cutters, but you should also get small detail friendly versions like this Hakko Micro Soft Wire Cutter for $5 on Amazon. Metal sheers are nice too and strangely called Aviation snips, here is a Stanley version for $9 on Amazon.
Vise-Grips style locking pliers are shockingly useful, here is a set by Irwin Tools for $32 on Amazon.
I hate Allen Wrenches because they seem like such a fantastic solution to the basic fastener problem, but are frustratingly small and slipper for anything but a 1/4 turn application -- and the world rejoiced on the 6th day when the Creator created the Hex Bit Socket Set, like this 32-piece SAE/Metric set for $54 on Amazon.
The basic household hammer is great for a few things...driving nails into boards...sure...but you need a proper selection of dead blow, 2-lb soft mallet, and 5-lb sledge to properly rebuild an Italian transmission. Here is a 6-piece set from Mountain for $46 on Amazon.
Metal files are always helpful for various projects, find a set for $20 on Amazon.
Somehow, I went for years without owning a grease gun, but if you've got older cars they are needed, here is a Lumax version for $21 on Amazon.
These little wire brush sets are really handy, be sure to get a set that has plastic, brass, and steel, like this ABN branded set for $5 on Amazon.
Obviously, you are going to need to clean your car, so various car cleaning supplies are needed; I've enjoyed using Meguiars products over the years, here is a Complete Car Care Set for $49 on Amazon.
A work bench is not complete until it has a bench vise bolted to a corner, like this Wilton 4-inch jaw version for $43 on Amazon.
The safest way (typically) to get under your car is with a set of stationary ramps like these Rhino Ramps offered for $34 on Amazon.
Ramps work great unless you need to remove a wheel, then you are going to need your floor jack, here is a 1.5 ton Aluminum version from Liftmaster offered for $80 on Amazon.
Finally, you are going to want at least 1 to 2 sets of jack stands, like these Powerzone 2-ton Jack Stands offered for $24 on Amazon.
You are probably going to have to deal with dead batteries, so you will want a decent battery charger, like this Schumacher SC-1200A SpeedCharge that is a microprocessor controlled charger with settings for lead-acid, AGM, and gell cell batteries, find it for $39 on Amazon.
Stay organized by putting nice labels on stuff in your garage, get a Brother P-Touch label maker for $26 on Amazon.
You need small set of powered hand tools, and I've had a set of Craftsman Nextec 12-Vol Lithium Ion drills, including the basic drill/driver, an impact drill, and right angle drill for years. I like the Nextec because they are compact, powerful, and inexpensive. You can get the basic drill for $63 on Amazon...but I think I paid less for them a few years ago, they might have been replaced by something better by now.
I've had a wall mounted bolt/nut assortment in my garage for years, and the original nuts/bolts are long gone, but I've filled the drawers with various left-over fasteners from my projects. Start with a kit like this 1,001 nuts/bolts/washers kit for $20 on Amazon and then slowly replace the bins with your own Saab specific fasteners over time.
This is only a starting list of stuff that should be in your garage, and I am probably missing all kinds of things that you consider essential to the garage -- what'd I miss? Blue nitrile gloves? Comments below.