Cheapest way to get yourself a flat workbench (essential for making sure wings etc are true) is to find yourself a solid core door and a couple of saw horses. You can usually pick up doors cheap at demolition yards (I got mine free from my local garbage tip).

Fairly essential hand tools are: a good sanding block (with about 3 times as much sandpaper as you think you'll need), a good (metal jawed, not plastic jawed) hobby knife with about 10 times as many blades as you think you'll need (it's amazing how fast balsa and iron on film blunt those blades), a metal straight edge (get one at least 3' long), a right angle (a plastic one from a drawing set), some string that won't stretch too much for aligning wings, pulling servo leads through tight spots etc (brick-layer's string is good), a razor plane, a razor saw, plus regular hand tools like drill bits & drill (hand or electric), a couple of screwdrivers, a pair of pliers to pend & cut music wire and a file or two to smooth the cut ends etc. It's also a great idea to get a pack or two of small plastic spring clamps from one of those discount shops to clamp things together while they set. If you're going to use iron-on covering you'll need at least a sealing iron with a cotton "sock". Lots of folks use heat guns (like blow dryers on steroids) to shrink it but personally I haven't seen the need yet. Oh, and allen (hex) keys... lots of them. Get a set of both metric and imperial. You'll want soft weights at some point to weigh down balsa sheeting while glue dries - you can do this cheap by filling some zip-lock bags with lead shot or even sand, and putting them in old socks.

Less essential but sometimes useful tools are things like dremels, drill press, bench vice. I don't actually use a razor plane - I have a very small carpenter's plane that does the job nicely. A pair of kitchen shears (like tough scissors) can be useful for trimming cowls and canopies. Oh, since you're working in an unheated garage you'll want a fan, both for cooling and ventilation (CA fumes are nasty). Probably the most important (yet often overlooked) bits of gear however is good lighting. I like to have two or three florescent light sources to reduce shadowing.

Salvage as many plastic take-away food containers as you can find to mix epoxy in - you should have plenty of mixing sticks from the kit off-cuts, but if not you can get ice block sticks from the local craft shops cheap enough.

Use your imagination and you can get a lot of this stuff cheap from demolition sites (lights, shelving etc), second hand shops (for hand tools), discount shops and so on. You don't need to spend a fortune at your local hobby shop - save the bucks for the kits.