Re: Riding habits
The best fabric for a habit is solid wool. Wool releases horsey odors and dirt/mud/sticks. You want to find a fairly light-weight, but firmly woven wool, in a darker solid color. The hems of riding dresses (and sometimes of dresses that are intended for generalized hard outdoor use, such as westward immigration) are often faced with leather--chamois weight. This takes the abuse of boots, saddle parts, etc better than a regular hem facing, and is slightly water-repellent; it also adds weight to the skirt hem, which is important when moving quickly.
There isn't a pattern out that I would actually recommend, though. Period Impressions often has problems with accuracy, drafting, and level of instruction; Heidi Marsh patterns often suffer from the same problems.
You might, however, get hold of some of the "Drafting System" and "Tailoring System" books put out by R.L. Shep (reprint), as they do have diagrams and such for women's riding gear, if I recall correctly. You'd be doing up a muslin version, and fitting it down, then using period techniques to construct it; you're more likely to get a good result working this way than by a PI or Marsh pattern.