1996 volvo 850 radiator started leaking from base after I replaced the PVC system. Is this common and will I need to replace the radiator now? Also what is the best way to clean out a second hand radiator?

 

It's best to replace the radiator with a new one so you aren't back in the same situation in a month or two.
If it's necessary to go used then using a "radiator cleaner or flush" would be the thing to do once it's installed. Just follow the directions.

Would be a good time to replace the thermostat depending on how old it might be since you'll have it nice and dry with the radiator out.

 

Not sure of the connection of your PCV repair to the leak unless you had to force removing some hoses. Most radiators fail along solder seams which can crack and produce slow drip leaks. Its a good time to inspect all hoses and the stop plug to make sure you're not seeing some leaking from a reinstalled hose. I totally agree with KaF that if you're going to do the work to remove the old unit, go new and don't worry about it for another 10-15 years. As noted, its a good time to do the big hoses, thermostat and the overflow cap (yes they can crack over time so its a worthy $10 part to renew). You may still want to do the flush prior to replacing the radiator to clean out the block and heater core as well. Just remember to turn on the heater to open the valves.

 

These cars do not have valves.