There are various Nokia models with netmon already installed. Later BB5 versions of Nokia don’t have it installed are require a .SIS file to be flashed to phone.
Many DCT3 and DCT4 have netmon.
My preference (although old model) is Nokia 3210. Battery lasts for ages, good sensitive and has many screen you like, I think. Also I told the version of netmon in 3210 being the first netmon-tester inside Nokia to be designed around internal antenna. Most MSs today are internal antenna.
As an alternative, you could try Alcatel 735 BE4. And look at the GSM Tracks and GPRS Tracks.