Suggest a good DSLR and lens for my need?

I'm new in DLSR world and have little to no knowledge. So please point me in right direction.

1. My budget for lens is around Rs.25000.

2. Subject which I mostly going to shoot

Portraits
Building and architecture
Landscape

3. I have selected Nikon D5100 is it going to be good enough for shooting low light images and fast shutter photograph

It will be very difficult to find a decent dslr for that budget, I would recommend reconsidering the money

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Increase the budget a little and you can get Nikon D3

The D5100 is an excellent low light camera and it has fast shutter speeds (up to 1/4000 second), but I'm not sure you'll be able to get one on your budget (Rs 25000 = about US $444), especially if you want to get a lens with it.

A D5100 body will cost you about $500, and with the kit lens you're looking at about $600.

Maybe you should look for one that is used/refurbished. That will save you some $/Rs.