Index and primary key

Hi,
I have one question :
I have a table, with 12 fields, and 70000 records (not more, that's a
thesaurus)
I will have to make a "select" only on one or two fields.
I have no index on this table.
And primary key is a field containing a five digit number (unique, not null),
which is an important information of the record (I made this field "primary
key" because I'm sure that it will be unique).

Do you think this conception is a good one ?
Maybe, I have to use one of the fields on which I will make a select as
primary key ?
Maybe I have to add an index field ? (if I understand, that's better-faster
to extract a record, but isn't my "primary key" 5 digit code similar ?

Thanks,
Gaetan