Now I 'may' be wrong, but this idea was mooted to operate off the back of another operator's AOC who coincidently operate the PA31-350 and have Middle East backing/ownership ?
As for the SD3-60, well thats a non-starter due to the cost of TAWS and TCAS for any of the scrap thats sitting around SEN/CVT etc rotting away, the wheat has been seperated from the chaff with all that junk and its unlikely that there would ever be another SD3-60 go into service in the UK as there just aren't any good ones left.
Be careful at this end of the market, believe me the work is more specialised than ever, on the Metro front the Spanish cannot be beaten on price and contracts are getting few and far between, ad-hoc is still around but less and less with the main player in the brokering world preferring to buy hours then 'manage' aircraft for their work with foreign operators than use one off charters with more expensive UK operators.
My genuine advice is that unless its underpined with a contract or two, there simply isnt the ad hoc work for that type of aircraft to go around.