Copper / Brass automotive heater cores are perhaps THE radiator of choice among serious WC people, they seem to offer one of the best balances between air and water flow restriction, and have excellent heat dissipation. Best of all they are far less (even when new) expensive than the specialty 'computer rads'.
For the absolute best results, get one that is a 'single pass' type core, (one pipe comes out each end tank) as opposed to the far more common 'dual pass' core. (both pipes come out the same end tank) A single pass core has less than half the flow resistance the same size dual pass core does. Oil coolers are far less effective as radiators since they are designed to work with a totally different fluid and temp range.
A heater core will offer more than enough cooling for a standard application. If you want to go semi-passive, some folks have been known to get regular car radiators. They are even better in terms of low flow resistance and cooling ability, but obviously their size makes mounting them a problem.
It is important to make sure that you get a rad / core made of metal that matches the rest of your system. for a Cu block get a Cu/Brass core, if using an Al block, get an Al core.
Airspirit over on Pro-Cooling has created a database of heatercores that allows you to search by cooling area size (note that this does NOT include the tank size, which will add ~1/2-1" on each end) at this location
As to new vs. used - Cost depends on what you choose, but most common heatercores are under ~$50.00 US brand new, so it isn't worth a lot to try to salvage an old one.
If I was to try salvaging, in decreasing order of cost and effectiveness:
1. Get it cleaned at a radiator shop (probably fairly expensive)
2. Try using a commercial radiator cleaner (from any autoparts store) although it might be a problem to circulate the coolant (make a mini-loop with your pump?
3. Flush with lots of water (garden hose)
I have also heard some folks use a solution of CLR cleaner for getting rid of mineral deposits and corosion and the infamous *sol enema to get rid of any biological / organic buildups.