The price for the oak is about par for the course for native oak but it does depend a bit on the quality. Kiln dried wouldn't be my first choice for outdoor furniture as it will have been kilned to 10%MC here in the UK-- in the US their white oak would have been kilned down to as low as 6%MC. In its exterior location it will achieve a low point of about 15% and could easily go up to 22%MC in very damp conditions. I'd prefer air dried material in the first place as it would be no lower than 15%MC and, more likely 16 or 17%MC when you buy it.
The best finish for exterior locations always causes debate. Clear finishes deteriorate very quickly if they're not maintained on a regular basis. Yacht/spar varnishes are good but they must be sanded down and recoated every couple of years at the most. Oil finishes are easy to apply but don't offer much protection from anything.
Sikkens make products for exterior use such as decking and these contain light blocking pigments. They might be worth investigating but, again, they're not maintenance free. In truth the best protective film for exterior use is one that blocks out all light and excludes air, ie, paint, but paint needs maintenance too.
Frankly, I think the maintenance of exterior hardwood furniture is more of a pain in the pants than just leaving the stuff bare. I've never yet met an owner of outdoor furniture that is willing to keep up with a proper maintenance and repair routine-- the job typically gets put off until next year, and the year after by which time it's always too late. So, in my opinion, unfinished and maintenance free is as good an option as any other-- this lets the oak develop its own natural silvery grey patina, and bare oak is very durable. Slainte.
