PVC has good UV resistance. It's not excellent UV resistance, but it stands up well enough that there isn't significant deterioration of the product during it's expected lifespan. Thats why they make windows and siding out of PVC.
Are you SURE that the plastic you saw turn brown and brittle was PVC? I expect it may have been either polyethylene or polypropylene, both of which have poor UV resistance. Or, if it was PVC, it could have been plasticized PVC. PVC can be plasticized so that it remains soft and flexible at low temperatures, like room temperature. Clear vinyl tubing is an example of plasticized PVC. It could be that the heat was baking the plasticizer out of plasticized PVC causing it to discolour and turn brittle. It's the heat that causes the plasticizers in vinyl (aka: PVC) to evaporate out of the plastic, and that's why vinyl dashboards in cars often harden up and crack. So, maybe what you saw was deterioration of plasticized PVC due to heat driving the plasticizers out of it, not deterioration due to the UV light from the Sun.
But, certainly any metal piping would be the most durable choice. Metals are bullet proof when it comes to UV light and normal outdoor temperatures.
The blindingly obvious question, however, is: Why lose the garden hose? It's flexible so that it's extremely easy to drain the water from your evaporative cooler into your vent stack. And, if it's ever damaged, it's easy to replace. It seems that whomever made your evaporative cooler felt that a garden hose would be the most practical way of draining water from it, so why lose the garden hose?
If you're existing hose works fine but is deteriorating due to UV exposure, you should be aware that you can buy garden hoses made of EPDM rubber, which has excellent UV and heat resistance. EPDM rubber is used as a roofing membrane on flat roofs because of it's excellent UV and weathering resistance. It's also the kind of rubber used to make the radiator hoses on your car because it stands up very well to heat. And, it's the most water resistant rubber available. If your hose works fine, but is deteriorating cuz it can't stand up to your heat and sun, you don't need PVC, or even metal... you just need a better rubber hose.
EPDM rubber - Wikipedia, the free encyclopedia
Heater Hose - Rubber Product - Elasto Proxy
Where I live, you can buy black EPDM rubber radiator hose in any automotive parts store. If you're fundamentally wanting better heat and UV resistance from the hose you've got now, I'd consider buying some 1/2 inch diameter heater hose at an automotive store and putting garden hose fittings on it. Then you'll have a garden hose made of EPDM rubber that will stand up to more heat and UV light than you can throw at it in a lifetime.
I don't know if radiator hose comes in 1/2 and 5/8 inch sizes like garden hose, but I know that both garden and heater hose come in 1/2 inch diameter, so you can make a garden hose out of 1/2 inch heater hose with 1/2 inch brass garden hose fittings.
You can also buy high quality garden hose and water hose made of EPDM rubber:
Garden/Contractor's Water Hose
If you opt for EPDM rubber hose, buy it from the places listed under "Rubber" or "Hose & Tubing" in your yellow pages. It'll be much cheaper at those places than at the auto parts retailers. It also occurs to me that for $5 you could take a box cutter and a set of 1/4 inch drive sockets to your local auto wrecker and collect all the 1/2 or 5/8 inch ID EPDM heater hose and hose clamps you need to do this project. The hose will be dirty, and you'd probably need to clean it up, but it still should be in usable condition.