Here are some threads on a tarantula forum about B. Smithi (You will want to start learning their scientific names, you would be surprised how much common names can confuse things...there are like 4 spiders people call the Texas brown, but they are all different species with different care. The mexican fireleg looks a lot like the red knee when they are babies. Know what you are buying!)
Big 'universal care sheet', edit to fit your T
This is a good description of their life in the wild, and a good help in determining how to care for them based on that:
A good tip with spiders is...Well, with these hardy ones, they don't need as much specific care. It's easy to over-worry about our fuzzy friends. as you can see, the temps the spiders go through are not super warm, but of course not super cold either. Average room temp matches their wild temps very well generally. They burrow in the months that it rains to escape that excess moisture, so you can see they appreciate a moderately dry environment. But as you can see, it's not a full desert environment... You don't need to match a specific number per say. They aren't quite as specific like reptiles would be. Watch your T--if your T is shying away from wherever the humidity is, maybe you made it too humid. If it is hugging the water bowl, maybe it's going to molt and it's simply TOO dry
It's not likely that your room will be 'too' dry though. Our natural humidity based on where we live is usually just fine for the dry species.
If you want an adult, it will be pricier, but you can guarantee sex(nice because males die young), but a baby is cheaper and you can raise it yourself. But they are slow growers, so it wouldn't be able to go into your enclosure for years.