With some ingenuity and a bit of scrounging you should be able to gather at least the basics for relatively cheap. You only really need 1 or 2 things from most of these pages, I've just tried to present a number of options.
I'll also note that this isn't really an Instructable, since it doesn't say how to build anything, but it's the best way I could think of to present a relatively detailed tool list for building plate armour. I also opted to use internet images after having a finished instructable, minus pictures, sitting for a month while my digital camera was, and still is, nowhere to be found.
Step 1: 1 Knowledge
What kind of armour are you building? plate, maille and cloth armours all use different supplies and techniques.
What is your armour for? is it decoration, costume, or will you fight in it?
If you fight in it what kind of weapons are you using, boffers, rattan, steel?
If your fighting with a group are there any armour standards or regulations you need to conform to?
Also, do you REALLY know what armour looks like? we all have a rough idea, but there is a lot of subtle shapes and details that separate a good suit of combat armour from something to stand in your hallway.
Start building a collection of images of armour, study them closely and really get a feel for the shapes that make up a suit of armour.
Books are also indispensable at the very least i recommend getting your hands on
Techniques of Medieval Armour Reproduction: The 14th Century ISBN#1-58160-098-4
It is the armourers bible, it covers everything you need to know about building armour, and then some. seriously, if you want to build armour get this book, read it and love it.
other than that just about anything you can find on armour will have something useful in it. even if it;s a simple and a picture showing some obscure detail you never noticed.
Step 2: 2 a Way to Cut Metal
here are a few ways to cut metal.
I personally use a combination of a jigsaw and tin snips.
Beverly Shear or harbor freight knock off
angle grinder and cut off disk
a hacksaw and hammer and chisel also have their place, but not so much for cutting out large pieces.
Step 3: 3 a Way to Make Holes
Personally, i use a hand drill and a knockoff whitney punch.
I'd have to say the most common tools for putting holes in metal are
whitney punch or knockoff
In a pinch I've also used a center punch over a soft backing to put a small dimple then used a file on the convex side to make a hole.
Step 4: 4 HAMMERS
here is a list of my commonly used hammers.
1 dishing hammer - built mine by grinding domed faces onto each side of a 3 pound mini sledge.
2 machinists hammers - 1 large and one small. I'd guess them to be around 2lbs, and 16oz.
3 ball peen hammers- large medium and small, weiging about 2 lbs, 1 lbs, and 12oz. My medium one had only got about a 6" handle on it. it's used for riveting in tight spaces.
1 dead blow mallet
and my less commonly used hammers
1 claw hammer
a full set of auto body hammers
1 wheelwrights hammer
1 bricklayers hammer
2 rubber mallets
a couple shingling hammers/hatchets
assorted hammer shaped objects.
All your metal faced hammers should have the edges rounded out and the faces polished. any ding or scratch in the face of your hammer will be stamped into the metal every time you strike it.
Step 5: 5 Dishing Forms
1 a stump.
a bowl can be carved in with a hammer and chisel, burnt in with charcoal and some time, cut out with a circular saw, tip of a chainsaw, or with a "chainsaw" disc for an angle grinder, ectera.
2 the bottom of a compressed gas cylinder
gas cylinders usually have a concave bottom. when compressed gas cylinders can no longer be re-used they get cut in half. asking around places that deal with them can usually get you the bottom half of one for the price of carrying it away. once you have one it;s recommended that you fill it with sand and weld a base plate on it. that way it's less likely to fall over, and doesn't ring like a bell. you may also need to grind out some lettering or some such on the dish area.
3 a swage block
A swage block is basically a chunk of metal with different shaped depressions, grooves and holes in it for forming different shapes or holding stakes. jewelry sized ones are easy enough to find, but locating one large enough for armouring can be quite another matter.
4 home or custom made metal dishes etc.
there are places out there that will make custom tools for you, or with a well equipped shop you can make your own. My main dishing form was actually made half by accident. I welded some 3/16" thick plate onto a piece of thick walled pipe, heated it up and bashed it down into a dish, then to add weight i melted down a LOT of tire weights and poured the molten lead into the bottom. after the first time using it the lead copy dropped out of the bottom. I found it easier to dish into the lead bowl than the steel one, so i started using it instead. when it gets to deformed i melt it down and re-cast it.
5 dishing doughnuts
think of a nice round doughnut, now make it out of steel and bolt it to your workbench. any good machine shop should be able to make one of these for you.
6 shot bags.
not the greatest for armour, but still useful for some things. it;s basically an industrial strength bean bag filled with steel shot, or lead shot, or even sand. you can find them in auto body shops for panel beating.
Step 6: 6 Stakes
A Ball stake
Mine is made from a 6lbs shot put with a hole drilled in it then wedged onto a length of 3/4" round stock. large ball bearings and trailer hitches are also handy for making ball stakes.
a T stake
I don't have a proper one, but i really wish i did. it would make curving a lot of plates much easier. a common way to make T stakes is to weld some beefy bar stock or thick walled pipe onto a shaft that you can clamp into a vice or mount on a stake plate.
there are about a million and ten other shapes and variations of stakes that you can find a use for. a lot of them will be pretty specific, but by the time you need them you should be able to design them yourself.
Step 7: 7 Miscellaneous Hand Tools
I'd reccomend getting...
a sheet metal gauge.
A fabric measuring tape.
at least 1 pair of good side cutters.
assorted types and sizes of pliers.
a hack saw.
assorted files and rasps.
a center punch.
cleco temporary rivets or small nuts and bolts.
assorted clamps (mainly C and bar clamps).
I shouldn't even have to mention it, but duct tape.