I will mainly need to use an audio interface to connect my keyboard, and want to have option for including vocals later – is Focusrite Saffire 6 USB a good choice for this ? mic preamps are needed for keyboard inputs ?

Keyboard inputs won't need preamps, though a good quality jack/socket input helps. Just use line-in for keyboard, and use mic in for vocals or other acoustic instruments recorded through mics. Focusrite preamps are among the best. If you choose a model with 2 preamps, later when you want to add mics to your tool kit, you will be ready to use two of them at the same time. Focusrite Saffire 6 USB Audio Interface is a good option.

Should I also get a cheap internal soundcard for the pc , or I should just plug the usb based external sound card each time?

I use a laptop. To watch movies and have a quick listen to songs, i just use the built in sound card 1/8" port. I don't want to switch on the MOTU sound card which i use for sequencer work, each time (external powersupply). In the case of Focusrite Saffire it may not be a big trouble though, because Saffire 6 USB is powered via USB, with no need for an additional power supply. Just plug in the USB and you are ready to use it.