'drug of choice' usually means the drug that you like taking - and usually refers to narcotics (i.e. mind-altering drugs). In this case, the sentence means that the writer thinks that having money would work as a drug to get rid of anxiety.

I tend to agree with Gary: 'drug of choice' generally means 'most effective thing for something'. However, the writer could be being deliberately vague, without seeing more context. The 'for' either means 'to reduce or lessen', but could also mean 'to enhance'. However, as drugs are taken 'to reduce or lessen' it probably does mean "having money would work to reduce anxiety".