Buying parents home with no deposit?

Evening all

I've been reading a few threads but thought it probably easiest to ask my question.. :-)

My parents are currently looking at relocating to the country with my brother and his gf. The idea came up today of me and my fiance buying their current house, however the challenge we've got is the deposit... We haven't really got any as yet,due to renting, etc.

The property hasn't been valued recently, but a couple of properties on the street have recently sold for around £180k... So that's the ball park we're looking at...
Working on an average deposit required of 10%, we would need £18k.
Is it valid for us to buy the property for £160k and use the rest of the property value as deposit?

Any gotchas, blocks or better ways?

Comments welcome, and thanks in response for any replies.

Regards
Gavin