Hi,
What's the behind the idea to build this with discrete parts?
The usual converter ICs are simple, cheap and most of all they work reliably, even for high baud rates. They usually include ESD protection and create output voltage levels according RS232 standard. A lso the input voltage thresholds are according RS232.
If you include all this in your circuit, then it may become complex.
There are even completely assembled very small and cheap modules availabke at ebay and many other places in the internet.
Klaus