Transmission going out or what?

Okay, I just bought a 2005 Ford Freestar mini Van4.2L v6 SE with a little over just 84,000 miles on it and "sometimes" the Shifter does not want to move out of the Park position, you have to try a couple times before it works. Now I read that it could be either the brake light switch or the Shift interlock Solenoid, However, the Brake lights do come on when pressed so I am ruling out the Brake light switch. However, when I am driving at normal speed and try to speed up, although I am almost flooring it, it does not gain any speed at all, sometimes the RPMs race pass the 3000 mark then slowly go down, but sometimes it goes to the 2000 mark and sits there a few second before going down. Same thing goes when I am taking off after being
completely stopped. And also the Brakes themselves are Very Bad. It has No power at all to it. Now I need to know if all of this is related to each other somehow maybe, and is it my Transmission going out or? When I try to speed up to pass someone, again it does not go any faster and my RPMs don't always go much higher. What do you suggest the problem is and where do I need to start looking? I am going to be doing the work myself because I can not afford to take it in. Thank you