What would be the best GPU setup if I have access to free electricity.
I guess it would be older cards that are not that power efficient anymore but are cheap.
Which GPUs and currencies would you recommend?
Equihash is a good option for some older Radeon cards. However, ZEC isn't doing too great right now, so if you're actually buying cards, be sure to run the profitability calculations for the specific ones you're considering. Zclassic is another equihash coin to think about.
In general:
Anything before the 900 series of Nvidia cards is probably not a great choice for mining.
For AMD, the HD 7000 series is about as far back as you should go. (Sure, a 6990 or upper tier card from the 5000/6000 series might be work for mining, but those are getting rare and the ones you would find have probably been heavily used.)
Do keep in mind that although you do not have to pay for electricity directly, you will need to purchase PSUs that will support the cards you select. (Which then increases your initial investment with respect to the hashrate you'll end up with.)
