What do the Pestilence and Inclement Weather armor effects actually do

No. The only official way to get the statue is to purchase the Legendary edition of Halo: Reach. However, there will definitely be some of the statues up for auction on ebay after the game comes out, but these will not be much cheaper then buying the game, and may even be more expensive.

Also, Bungie has stated that the Legendary edition (including the statue) is going to be very limited (as opposed to the Halo 3 Legendary edition which they are still trying to sell). There have already been a couple reports of retailers no longer selling copies of the Legendary edition because they are sold out. Nothing official has been said about whether all the copies are sold, but if you really want a statue, and don't want to press your luck on ebay, you should look for a copy soon.

One other option you have is to buy the legendary edition, and then try and sell the game. If you have a friend that wants the game, you could probably sell it for $50 (not sure of your country of origin, but in the US the limited edition of the game is selling for $70 new). You can also trade it back to Gamestop (or another used game retailer), but you will not get very much for that.

Also, the Legendary edition comes with a redeemable code for in game flaming helmets (and a couple other in game items). This alone could most likely be resold for $30-40 on ebay, because these are very desirable in the Halo community.

Really, you have two options. Buy the Legendary Edition and try to sell the stuff you don't want (or keep what will be a great game), or wait and try to buy the statue after the game is out.

Well, now according to the Halo Wiki the Mark VI helmet costs 300,000 cR and is unlocked at Brigadier, which itself requires 1,400,000 total credits. I suppose next month we'll see if that's accurate.