@Grant Lin
Ran into the same problem when replacing an all carpet home with wood flooring. Did the same research you are doing now and found that Lowe's carried a red oak RetroTread for about $30. The other idea was to build the staircase myself with material from the lumber yard.
While the lumber yard would have been cheaper I was questioning my ability as well as speed. This was after all, my first flip. I was already behind my original schedule. Therefore, I chose the RetroTread from Lowe's and purchased the risers from the lumber yard.
Regarding top of the staircase, I used a tread and then a reducer transition piece between the tread and carpet. Wood downstairs and carpet upstairs.
Looking back or if I do a similar project in the future I'd save the bucks from Lowe's and invest the money in more time to build the treads myself. I say this due to the confidence built during the job. But in the beginning standing at the bottom, the stairs looked like Mount Everest!
Attached are photos before, during and after. All work done in house.