Actually, you take the Con score of the new form also. Now, is it just me or is there nothing in the description of either Wild Shape or Polymorph that says your gear merges with your form or anything like that. So, sure, if your gear no longer fits you it falls off, but if it does...
20th level characters are expected to have 760,000GP worth of stuff. so let's gear up!
Since you're a Druid and you Wildshape all the time, the only stat you care about is Wisdom. With a standard 32 point buy this gives us starting abilities of:
Str 10
Dex 10
Con 14
Int 10
Wis 18
Cha 12
Given 5 inherent ability score bonuses from levels that brings us to Wis 23.
With character wealth we buy a +5 Tome of Understanding bringing our score to a more respectable 28 (cost 137,500gp). Remaining wealth = 622,500gp. Tack on a 8,000gp Incandescent Blue Ioun stone for +2 enhancement bonus to Wisdom and we're up to Wis 30, which is about where we want it. Remaining wealth = 614, 500gp.
Rings are nifty if you WS into Dire Apes or Chokers a lot because you'll get to keep wearing them. But the only ones that are really interesting are Counterspelling, Spell Turning, and Three Wishes.
Let's go in for a Monk's Belt though. That'll give us +11 to AC in any form for a bargain price of 13,000gp. Remaining wealth = 601,500gp.
Druid's Vestment costs 10,000gp and is the only core method of gaining additional uses of Wild Shape, albeit, just 1 extra. But we'll take it. Remaining wealth = 591,500gp.
Scarabs of Protection, 38,000gp, are useful since they grant some immunity against the nastier effects in the game and because you'll still get to wear them in wild shaped forms. Just pick up one for now and save some gold (you aren't in especial need of gear). Remaining wealth = 553, 500gp.
Anything else you buy will be for your normal form, and any humanoid, and/or vaguely human shaped forms. You could invest in a Ring of Protection +5, Bracers of Armor +8, and a Periapt of Wisdom +6 if you choose (total cost 50,000 +64,000 + 36,000 or 150,000gp). You might also buy a few Pearls of Power for extra spell slots per day. If you bought one for 7th level spells, one for 8th, one for 9th, and one for recalling two spells of 6th level or lower, as well as the other items that'll take you down to a remaining wealth of 139,500gp. Not too shabby.
Let's begin the build.
Since you don't care about feats as a Druid, let's go ahead and be a Dwarf, because +2 to saves vs spells is nice. The boost to Con is welcome in your natural form, and the penalty to Cha really doesn't matter. As far as feats go Combat Casting and Improved Initiative are musts, as is Natural Spell. That takes up our 1st, 3rd, and 6th level feat slots. Power Attack might come in handy while in a powerful form, but your DM might not let you take it because you only have Str 13 while Wild Shaped. To get your spells' save DCs up though you can take Spell Focus and Greater Spell Focus a few times. You don't really need other feats, except maybe Heighten Spell to use with Baleful Polymorph (though it's somewhat unnecessary). But Finger of Death will be your bread and butter when going the Choker route, so pick up Spell Focus (Necromancy) and Greater Spell Focus (Necromancy) at the very least. The other feats are up to you. The save DC for your Finger of Death will now be 32 if you're in natural or Choker form, 30 if you're a dinosaur or assassin vine.
As far as spells to prepare, your 9th level ones aren't very good, so 4 Shapechange seems fitting. You can use the extra slot for Storm of Vengeance or Antipathy (or something). 8th level has a few nice spells and you have 2 bonus spells from a high Wisdom, Finger of Death is great, so you want 4 of those, but with the other slots look into Word of Recall, Reverse Gravity, or Repel Metal and Stone. At 7th level, Control Weather, Creeping Doom, Heal, and True Seeing are all good. At 6th level Greater Dispel Magic is always good, and Find the Path, Fire Seeds, and Antilife Shell have their uses.
Here's a character sheet, for easy reference:
http://www.myth-weavers.com/sheetview.php?sheetid=245387EDIT: I had to scrub a couple totally awesome tactics because Druid spells completely suck. Anyway, everything else above still applies.