Pretty much any supplier we deal with sells some kind of camera system, so you should be able to find a system from one of your suppliers.

As others have mentioned, I would replace the cameras with new as well. You are just asking for trouble if keeping the existing cameras installed and it will be a matter of time that the first one dies and needs replaced.

If wanting to utilize the existing wiring there are many TVI cameras that are fairly cost effective and will work with either a TVI or 4/5 in 1 recorder. The customer will be much happier when they can actually make out what they are looking at with a new TVI camera. Find a recorder with multiple HDD bays (most 16 channel recorders accept at least 2 HDD) and buy the stand alone unit (without hard drives installed). Then find the largest size surveillance hard drives it will accept (most are 8-12TB/slot) and load up the bays, install the system and walk away.

For our cost effective systems we use Galaxy (HikVision OEM) cameras and recorders and have little problems with them. The app works for what people need it for and is a cloud based system so no need to modify any networking. For security and better options we get into Samsung and Axis, but these come at a higher price point.