I'd use a C20 mix. I bag cement to 2 of sand and 4 of aggregate. For 1 cubic metre, you'd need 320kg cement, 640 kg sand and 1280kg of aggregate (10-20mm). Oh and about 300 litres of water. For that quantity it's worth considering a readimix delivery. I've never been impressed by the 1m3 isolated cube for a pier foundation argument. When I eventually get round to building my own, I'll just pour a 150mm thick slab for the whoe thing and build a pier from concrete blocks on top of it. ETA Oh and I'm on glacial till i.e clay with rocks in it.
If you do decide to buy rather than build, I'd recommend the Davis Vantage Pro 2. I've been running one for years and apart from 2 sets of anemometer bearings, it's been faultless. There are people on the various forums (Cumulus, WXForum) that make aftermarket data loggers for interfacing to Cumulus. The initial outlay is high but the reliability compensates for that.