SAAB 340 CROSSES FAF ON NON PRECISION APCH BELOW PUBLISHED MINIMUM ALT AT LAW.

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WE WERE A RESERVE CREW ASSIGNED TO REPOSITION AN ACFT FROM DFW TO LAW UNDER PART 91 (WE HAD NO PAX). WX HAD BEEN 1/2 MI VISIBILITY AND 100 FT CEILING FOR MOST OF THE MORNING. NORMALLY THIS WOULDN'T HAVE BEEN MUCH OF A PROB; HOWEVER; THE MAIN RWY IN LAWTON IS CLOSED FOR CONSTRUCTION AND THE PARALLEL TXWY IS BEING USED AS THE RWY. THE ILS HAS BEEN DECOMMISSIONED FOR THE DURATION OF THE CONSTRUCTION. THE ONLY APCH AVAILABLE IS THE VOR RWY 35 WITH CIRCLING MINIMUMS EVEN IF LNDG TO THE N. THE RPTED CEILING WAS 500 FT OVCST 10 MI VISIBILITY WHEN WE ARRIVED; BUT SINCE WE WERE PART 91 AND CATEGORY C CIRCLING MINIMUMS FOR THE SAAB ARE 1700 FT MSL (590 FT AGL) WE DECIDED TO MAKE AN ATTEMPT AT THE APCH. WE CROSSED THE LAW VOR AT THE PROPER ALT AND THEN DSNDED TO 1700 FT (MDA) AND STARTED LOOKING FOR THE FIELD. WHEN WE WERE AT THE 3.5 DME VDP WE COULD SEE THE FIELD BELOW US; BUT IT WAS APPARENT THAT WE WERE TOO HIGH TO MAKE THE LNDG. WE EXECUTED THE PUBLISHED MISSED AND PROCEEDED TO GO TO THE LAW VOR TO HOLD. WE INFORMED THE TWR OF OUR INTENTIONS TO HOLD OVER THE VOR SINCE IT APPEARED THAT THE WX WAS RAPIDLY IMPROVING. WE WERE HANDED BACK TO FORT SILL APCH AND WERE INSTRUCTED TO HOLD AT 4000 FT (WE WERE VMC ABOVE 3000 FT) TO LET ABC TRY THE APCH. WHEN ABC ARRIVED; THEY SHOT THE APCH AND GOT IN WITH NO PROB. WE TOLD APCH THAT WE WOULD LIKE TO TRY THE APCH AGAIN AND WERE CLRED FOR THE APCH FROM THE HOLD. WE TRIED TO KEEP IT AS CLOSE AS POSSIBLE TO THE VOR (FAP) TO SPD UP THE APCH. UPON ENTERING THE CLOUDS; WE RECEIVED A SINGLE GPWS ALERT 'WHOOP; WHOOP; PULL UP.' COMPANY POLICY AND GOOD SENSE REQUIRES A CLB UNTIL THE WARNING STOPS. SINCE WE WERE CLOSE TO THE FAP AND COULDN'T MAKE A STABILIZED APCH AT THIS POINT; WE ELECTED TO GO MISSED ONCE AGAIN. WE INFORMED ATC THAT WE HAD RECEIVED AN ANOMALOUS INDICATION ON OUR INSTS AND WOULD LIKE TO GO BACK TO THE VOR AND REATTEMPT ANOTHER APCH. WE WERE GIVEN A VECTOR AND A TURN TO INTERCEPT THE INBOUND COURSE AND THEN CLRED FOR THE APCH. UPON INTERCEPTING THE INBOUND COURSE; I MISTAKENLY TOLD THE FO (PNF) TO SET IN 1700 FT INSTEAD OF THE XING ALT OF 2400 FT. THE FO DIDN'T CATCH THE MISTAKE AND SET IN THE INCORRECT ALT. WE BROKE OUT OF THE CLOUDS ABOUT 2400 FT AND CONTINUED DOWN TO 1700 FT. I DO NOT REMEMBER AT WHAT ALT WE CROSSED THE VOR. IT WAS PROBABLY BTWN 2400 AND 1700 FT. WE WERE IN VMC AT THIS POINT AND THERE ARE NO OBSTRUCTIONS IN OUR WAY; HOWEVER; AT A DIFFERENT ARPT OR ON A DIFFERENT APCH DSNDING TO MDA CAN AND HAS CAUSED DISASTROUS OUTCOMES. BEING DISTR BY THE GPWS AND NOT REFERRING TO THE APCH PLATE PROPERLY WERE DEFINITELY CAUSAL FACTORS. THE FACT THAT I HAD A NEW FO (76 HRS IN TYPE AND 1 MONTH WITH THE COMPANY) WAS CONTRIBUTORY. HE WAS EITHER NOT IN THE FRAME OF MIND TO QUESTION THE CAPT; OR HE WAS 'BEHIND THE PWR CURVE' WITH ALL THE GPWS AND MISSED APCH ACTIVITY.