Hi
If your building a computer make sure you get Kontakt 4 http://www.native-instruments.com/#/en/products/producer/kontakt-4/ as it is one of the most versatile VSTi around. (Reads samples from other manufactures as well)
Also if you want a plug-in host with hardware control (Plug-ins and Midi) have a look at Kore 2 http://www.native-instruments.com/#/en/products/producer/kore-2/
There are many other VSTi so as far as a home studio is concerned, the sky’s the limit.
Hope this helps

Bill

BTW if I went for a separate keyboard I would go for a Kurzweil, (Always said I was going to get one, but never got round to it) but this may be out of your price range.