VAT Voluntary registration to claim vat back on van

Hi 

I think i am having a bit a moment here, my mind has gone blank.

I have just received a call from a client advising that he is looking to buy a van for £17,000 including vat, but he is wanting to volunteer to go vat registered so that he can claim the vat back, i advised that he would have to then charge vat on his sales going forward, so he may not save in the long run, to which he replied how long do i have to be vat registered before i can de-register again!

To me it looks like anti avoidance, 

Does anyone have any advise on this.  I would suggest not going vat registered as the who amount can be expenses anyway!!!