*Amateur leatherworker raises his hand*
As for holsters for weapons, there are actually guides (believe it or not) from Tandy Leather that give some info on some useful guides on making holsters. In particular, some of Al Stohlman's guides (in particular volumes 1 and 2 of
The Art Of Making Leather Cases) would be potentially useful--volume 1 covers a lot of the basics, and volume 2 does have some guncase designs that may be adaptable to swords.
Strapmaking is less difficult than it sounds--if you want to get fancy and have fancy braided strapping, there is an excellent book by Bruce Grant entitled simply "Leather Braiding" that would work wonderfully; otherwise, I would suggest getting some heavy oiltan leather (maybe 8-10 oz thickness) and one of the larger strapmaking tools at Tandy or Osborne Tools. (You can also get precut cowhide of the necessary lengths, and it's likely to be cheaper, unless you also use the same hide for the armour.)
For the armour (and the cases, and the straps) you'll want 8-10 oz leather or heavier (this is actually a measure of thickness--an 8 oz thickness being roughly equivalent to 1/8" thick leather; there's a pretty good conversion scale between "leather ounces", "leather irons" (used in the shoe trade), and fractional inches
here that also includes millimetre lengths). I've not seen books on leather armormaking at Tandy, but you can likely find some guides on it on Amazon--many of the techniques for making the armor are similar to making leather cases, and some similar to shoemaking.
As for decoration...well, the best question is whether you intend to carve or stamp the leather. If you're going for medieval look, technically carving is more "period", but both can work; if you go this route, I would recommend getting a copy of a Celtic patterns book and Stohlman's "How To Carve Leather" (you can find both at Tandy and similar leather stores fairly easily). Also be prepared to invest in stamps--at minimum you'll want some nice beveling stamps, both smooth-edged and with stipples on bottom (if you need, feel free to private message me on my LJ account and I can give you some specific recommendations as to basic stamp tools for carved leather).
Staining is fortunately not the pain it once was--eco-friendly dyes and stains do exist and give nice contrast, and there are techniques to give different colour to different parts of leather (again, PM me if you need advice on techniques).