Before doing anything flush the system and get as much crap out as possible. Easier said than done, this could take hours, but well worth it. Back flush everything if you can. The radiator is likely to need replacing on most cars though as they just slowly die from day one if not correctly maintained.
I actually do pressure wash alot of coolant systems, but you have to be careful, depends what you doing i suppose. But it always amazes me how much crap comes out when you do that!
If running on road i would always be tempted to run Water/Coolant. Keep a good healty mix based on the time of year, flush it out every year or so.
The other great thing about water is if you get a leak away from home, you can usually get hold of water easily and get yourself home.
Waterless coolant systems yes are supposedly better, but the cost out weighs the improvement in my eyes. You also need to make sure the system is properly conditioned and flushed and cleaned etc before using. What a pita.
If the coolant system is working as it should, then no need.
If you are abusing the car because of the way you are driving it and the mods done and for long periods(e.g. race track) then yes the effort or running waterless starts to pay off, but this is the only time i would consider it personally.
Even on my race car buld however i am going to try and use the standard water stuff though as my cooling system should be more than adequate
Plans have changed to just make a good fun Track Car