How can I convert from 12v serial to 5v serial?
The signal level must be converted and inversion must take place. 

2mbaud is pretty easy (except it's a bit slow for most hardware and firmware), but if you actually meant 2Mbaud, that could be a problem. 

There are some transceiver chips such as MAX3245E that can typically reach 2Mbaud with a short cable, but they are guaranteed only for 1Mbaud. 

Seems to me like you'd be better off keeping things at TTL level by getting a CH340 or FTDI chip.