Okay you two!
I’ve got some links for you guys. My advice first is to develop them like any other character, they have a background personality ect.
If I was you I would also do a lot of research into organised crime. Look into how crime lords organise these hits and this should help you out a little.
Also as for clothes, I think they’d wear anything they could to blend in. If it’s day they’d probably wear casual ‘normal’ attire and at night something darker to blend in. Weapons you will need to research. I couldn’t find much for you on specific weapons they would use but I believe they’s use whichever weapon would be easier.
When looking into the guns check how many shots it can fire, people always get confused at reloading.
Hope this has helped,