Don't forget these games are pretty old. Yes, there is no way in both games to switch difficulties. Therefore you have to play Silent Assassin at least 3 times + some extra missions to collect missing weapons.
If you want to minimize the effort you should start Contracts on Pro.
Unlike Silent Assassin all difficulty related achievements in Contracts do stack and you'll need a silent assassin rating on ALL missions in Contracts anyway to unlock all the weapon achievements. So start on Pro, collect as much weapons as you can find (the guide is missing out all the bonus weapons you'll get for SA rating in Contracts), finish everything with a SA rating (with videos on yt it's quite easy) and you've done almost 90% in one playthrough. Then just go back collect the missing weapons, especially the melee weapons and you're done.