Which law Dictionary should I buy?

I am a fresh Advocate working in Calcutta High Court, India. I am in need of buying a Law Dictionary. I want to buy a good one,which would help me throughout my life…..of around Rs. 5,000/- or more may be. But I dont know which one to buy. A book-seller had offered Law Lexicon. But somebody told me that a Law Lexicon is not a Dictionary in its true sense.Strangely, i didn’t find the meaning of the term ‘Resjudicata’ in it ! So, i am left undecided. Can anybody tell me any guiding principle of judging a good Law Dictionary? Or, can anybody suggest a name?