I've been doing some reading and Dash Command seems to be well liked for the app side, there's some compatible "dongles" on their website: http://palmerperformance.com/hardware/
Check out Wasso's post (post#205) - http://www.350z-uk.com/topic/92221-ipad-mini-install-obdii-dash-command-page-10/page__st__180
From what I've read, lots of people have had success with Amazon/eBay ones for between £7-£20 (other reviews show they can be hit and miss). If you buy from Amazon, it's usually simples to return something if you have issues: http://www.amazon.co.uk/gp/aw/s/ref=nb_sb_noss?k=wifi+obd2+elm
Then there's a bunch at about £60-£120, like - http://www.kiwiwifi.co.uk/product_info.php?name=PLX-Kiwi-2-wifi-for-iPhone&model=kiwiwifi&prod=PLX-0029&osCsid=0bbbec04605d280004739bd1cee84559
Then you start to get into bigger investments, the wireless dady appears to be - http://www.espautomotive.co.uk/smartphone-devices/obdlink-wifi-scantool-works-with-ios-iphone-ipad-p-57.html
From what I've read, I'd say take a bash on a cheapo wifi unit for dash command. If it works, you're in, if it doesn't you're only £10-£20 down. Or go balls deep on a higher price one and hope they've done a good job!