Heat will be an issue if you have the GPUs in an enclosed space such as a case or confined area. You can use riser cards to extend cables so they are not in confined area and do not use a case if using air cooling unless you are sure you can build a good system with enough ventilation. If water cooling is out of your price range then you can use a desk fan for additional cooling of 10c-15c with additional noise levels. I would go with a higher powered PSU because you will get more use out of it in future either way or sell it on. Otherwise buy two cheaper 80+ rated PSUs and use build or buy something like "Add2PSU". Be prepared for heat, some people put their rigs in basements, garages or similar where it is cooler but check with manufacturers for minimum temperature range to avoid damage to capacitors, etc as excessive low temps may cause damage. Condensation is not a risk other than with liquid cooled systems in a warm environment, if doing that then ensure the cooling is not cooler than ambient room temp because you will cause condensation to build up.

You won't need much RAM, 4-6GB of RAM should be ok and CPU can be cheap low powered on and any boosting or OC can be disabled for CPU if not using PC for any other use than mining.

Edited for corrections and additional info: be sure to not allow GPUs to have temps over 90c as this will cause damage, try to keep within 75-80c range if possible while using air cooling.