1)if we make our own board then we would need to write our own OS is this correct. I can make the board thats not a problem, but i have never written a OS for such a thing. If we get a board readymade then we do get IDE and its basic OS.
you'll only need an OS if you are wanting to do very fancy things but a microcontroller can run a single program without needing an OS.
you will need a compiler, programming language and libraries for your programming language.
compilers, programming languages and libraries tend to be written for the microcontroller rather than the development board.
as an example, a lot of people on the forum use AVR microcontrollers.
there is a GCC port available for AVRs. once you get a programing environment working for AVRs you will be able to program any AVR based board using it.
2) what are the basic things in a microcontroller i should look for.. serial ports, ADC interface, memory. etc...
yup. most basic microcontrollers will have the functionality you have described. for your reference a microcontroller will only implement a logic level serial port (ie. +5v) and will be called a UART (or USART) so will need a little extra circuitry to connect it to a PC serial port.
3) now a days 32bit microcontrollers are pretty cheap, so it best to go for it.
to be honest, as a first project i wouldn't worry too much about the raw power of your microcontroller. it makes little difference for simple projects.
Admin talks microcontroller Noobs through the basics in his $50 robot project: http://www.societyofrobots.com/step_by_step_robot.shtml
whether you choose to build it or not it would be good background reading for you.