An expert frame should fit you unless you are oddly proportioned and most have a weight limit around 125 lbs so you could use it for a while.
Just wear jeans to start and ride flats until you are comfortable on the bike. Clipless will set you back quite a few bucks as you need the pedals and shoes. Changing the seat on an expert race bike is not money well spent as you would rarely sit on the thing and it will add weight to the bike as the seat and post these bikes come with is very light. If you do change the seat you would need a new seat post as the stock seat and post on the smaller bikes are riveted together, there is no clamp on the post.