I bought my mask new.
Find something that you like the look of and something comfortable (remember that you will be running in it!)
I found that even with single lenses fogging doesn't have to be a problem. I have tried various anti-fog sprays (both those sold for motorcycle helmets and those specifically for paintball lenses) but in all honesty, the best solution I have ever had was washing up liquid!
1. Add to inside of lens with finger (thin coat will do!)
2. Wipe around the surface
3. Buff of with a cloth/kitchen paper
You will find even the cheapest new goggles & lens better than those supplied on a rental site - it hasn't been used by hundreds before you poking their fingers on the lens, spitting on it, wiping and cleaning etc
As you go up in features (the lens itself - anti fog coating, dual layers etc) then the performance improves
Just keep an eye out in the sales posts! They come up and you can usually find what you want it's surprising! I haven't met anyone who isn't friendly so if you see someone with a pair ask to try them on when you finally get time to go!
Can't agree more with everybody else. Get a mask first then everything else will fall into place once you've been to a few walk-ons and found out what gear is out there and maybe trying some of the guys guns out and asking lots of questions