Regular briefs as well as thermal insulated suits (tops & bottoms) are great to have on hand for the cold winter months. In fact 2 sets are not a bad idea. You can layer them if needed or wash and dry one set while wearing one set. Polypropylene is a type of material some of these items are made of. Excellent insualtion factor and ability to keep you warm even if wet.
Your outer pants should also be of synthetic material or a synthetic blend.
Your pants & shirt need to be of the Basic Duty Uniform type (BDU's) found at any army store or police supply houses like Gall's. Get them a little bigger than what you actually need so when you "bulk up" during the winter-they still fit.
Generally they are made of 65% polyester and only 35% of rip stop treated cotton.
Rip Stop give the cotton fiber strength and a plastic like coating that sheds water.
BDU's also have some other very useful features like draw string closures around the wrist and ankles to keep bugs and cold air out, warm air in. Multiple pockets, no zippers to break or snag, no cold zippers for your fingers to stick to in the cold. Can be easily repaired with a needle & thread.
If your working in tall grass, weeds, or wooded area's you may want to get some gaitors to tie around your lower legs to keep the moisture off your pants.
Rain gear should be of heavy duty construction. If it tears easily or gets holes in it easily-it probably will not work in the environments and conditions you'll be working in.
Military poncho's are a good quick rain cover that can also be utilized as an emergency "tent-shelter", hammock, or sun shade. It can also be used to collect rain water for drinking using the hood portion to fill your water bottle ot cup.
Full suits of raingear consisting of pants and coats with attached hoods are also very effective.
