Plain cloth [wool, cotton] fall front trowsers will be fine, available from Crazy Crow, Jas. Townsend, etc., for not too much money. Don't jump in and get leather clothing right away. Leather can be expensive, and correct [braintan] leather is even moreso.
Both companies sell decent shirts, they're available in plain white, solid colors, or checked--your choice. If you've done CW reenacting, wear your Jefferson brogans--no need to get moccs until you know what you want. Some kind of hat or cap is good, but nothing wrong with bare-headed, either.
A long rifle is just fine. Don't worry about a Hawken--unless you just have to have one (I know how that goes :wink: ), as the common longrifle was the gun of choice anyway. Do you have a bag & horn? No need to get fancy there, either, in fact plainer is better.
A simple narrow (1 1/2 inches or so) belt with a plain buckle, with a common butcher knife and sheath will be best. Don't worry about a capot (another expensive item), just wrap up in a blanket for the time being.
What I'm trying to do here is point you in the right direction for reasonably correct clothing and gear without too much initial outlay. That way, you can get your feet wet and find out if you like the fur trade era. It'll give you a good basis to expand on if you find you want to continue, but you won't have a lot of useless or incorrect gear if you decide to progress.
Rod