"Right now two available choices to me are - Keil and Tasking."
Keil, Keil, and if too expensive for you then HiTech

"Also once my software is ready, how do I test it with my actual target system harware? Is there any general purpose board of such a type is available, where I can download my hex file directly from PC and simply give go command from PC?"
Ceibo EB-51 available from Philips (lowest price) and Ceibo.

"I would prefer hardware kit also suitable for many derivatives of 8051, just like software tools if possible.
I would prefer to have everything from a single vendor."
Do not worry about single vendor, Ceibo has a "keil" mode.
Have fun with your new toys
Erik