Hi Matty389 welcome to the forum!
I would highly recommend a Garrett ace 250 as a starter detector. It is very easy to use, and finds all sorts. I have this detector and have found loads of coins and artefacts with it. If you get it new you will get 3 years warranty with it. I wouldn't get an expensive detector at first, as you need time to get used to a detector, learning what signals to dig and not to dig. Theres loads of detecting video's on youtube that will give you an idea of what to do with a Garrett.
A local club is the west Kirby metal detecting club, heres a link to their site.... http://westkirbymdc.co.uk/
I would defiantly take your dads friends offer up on those fields, you will have years and years worth of detecting on them!
I would love to come along with you but im off my feet at the minute with a broken leg
If you need any more help just let me know