Just avoid the Heineken kegs (and Newcastle) that include their own tap. There is an unremovable plastic CO2 charger inside the keg itself. You want the brands that use kegs with a gravity dispensing tap. Clean them correctly and all you really need is a new bung for the top.
What you do from there depends on a few things. Do you want to dispense using CO2? How quickly will you burn through 5L once tapped?
If you do want to go the CO2 route, and put these in your fridge, then I'd probably recommend a DIY tap like can be found on the Northern Homebrewer forums. More flexible. Can use something like the Tap-a-Draft, but I found the system to be a bit fragile from all the plastic, and the fill speed is pretty slow when drawing a pint.
I've decided to move to a small keg setup (2.5g kegs) instead of playing with the mini kegs more, but if i had to do it again, it would be with the 5L kegs and a DIY tap.
