Should I buy a new car of get a new transmission?

I bought a 2003 ford escort in February of this year, and it has had some problems. I believe it has the biggest problem of all; needing a new transmission. The check engine light has gone on and off several times, and it always happens when I leave my car parked for a long time, and when it first shifts into gear. It came back on, and my boyfriend used the computer that reads the codes to reset it, but before he reset it he read me it gave off two codes: PO175 and P0000. The way he explained it to me, it doesn't sound like it'll only cost a few hundred bucks to fix. I believe I have a while before the transmission completely goes out, because it still is able to shift into gears, it just shifts very hard. Obviously buying a new car is so much trouble, so would it be more worth it if I just took my car to a co-worker of mine who fixes cars, buy a new transmission for him to put in? I'm thinking that I can buy a new transmission, and throw him a hundred couple bucks to put it in. So should I do my research on finding a new transmission, or should I just bid farewell to the car? Should I just drive it until the transmission goes completely out, then get rid of it? It has 96,xxx miles on it, and I believe I can drive it for another 20-30,000 miles, since I drive to school 25 min everyday, before it kicks the bucket. Any advice is appreciated!