Sir Griff,
The first thing to do would be to decide if this armor is for combat, or display. A lot of times,
you can get a suit to do both by purchasing "beefed-up" items( gauntlets,etc.) as extras.
After that, picture what time period you're looking for. Then, get the idea set in your mind.
If you are just purchasing "half-armour" you already know what you won't need, right?
You might want to purchase, or make, (as I'm doing) your arming vest or doublet after you
purchase your cuirass or breast-plate. That way, you'll know where you will have to tie,(or point)
your vest to your plate. As far as order, there really isn't a set guide as to what to purchase
first. I DO recommend using the same armoury or armorer for ALL your pieces since armorers
have a tendency to create different finishes on their product....Wouldn't want a satin finish
thrown with a high-mirror polish finish....
I used to have a list of several armouries in Poland, Czechoslovakia, etc. but lost them when
my computer went down. Money-wise, I'd go with Armstreet out of the Ukraine if you don't mind
a bit of machining to fine tune your rig. The cost for my suit was very good for the total package.
I will just have to supply my own undergarments and shoes(boots) and I'll be good to go. They had a pretty good delivery time here to the states so I don't think you'd have a problem. Good luck with
your project and let us all know if you need assistance.