I bought one a year ago from a gardening /hardware type of place it was around �70 and it works ok. I had not used a metal detector before then or since so I have no way of comparing it with other models.
I chose a sand comber one as I wanted it for the beach I have found / earthed many tin cans on the beach and horse shoes in a field lol.
I think features to look for would be a headphone socket, the most dials possible - mine only has one has shows me the strength of the signal to help signify depth or size of buried item. Batteries are important mine take AA which are very easy to get my hands on and very inexpensive, but I have not had to change the batteries once yet lol. A way of checking the batteries are low though is important because you dont want to be walking round for an hour with low batteries so it isnt picking any signals up ...whoops.
I think a big thing is comfort I did not really think to much about this and you do need it. I get cramp in my arm and back etc - repetitive strain injury from carrying it round - it doen't weigh much but after an hour it does become uncomfable. You can buy ones with not only a handle but an arm support like cruches have I think this would help carrying it. They should have extendable heights so if your short or tall you just adjust it. Mine also dismantles so I can fit it in a big rucksack.
http://www.thenaturalworld.com/NaturalWorldSit e/pages/product/product.asp?prod=2615000&ppcse id=3240&ppcsekeyword=metal+detector&cookie%5Ft est=1
This shop is on the high street too and is found in shopping centres mainly for some reason.
If you do get one ...lag out a carpet or rug and hide coins 2p 10p etc underneath and let him find them