Verus wrote:Is this build improved at all by polymorph or shapechange?
It's not terrible. As a bonus you don't really need Augment Form feat. In fact Augment Form is only worth it if you are going to primarily use Shapechange. The feat just makes your low BAB classes in to Medium BAB, thus instead of having 25, you'll have 27 BAB, but only when you are in an alternate form, via Polymorph or shapechange. In short I would say it is NOT improved, but there is no harm in using Shapechange now and again. However, Shapechange was designed/balanced to be a useful spell for standard casters. It wasn't designed to empower Gish builds. Thus you'll be fine with it, but I would not say it's going to be good as your normal form, buffed up and bashing.
Verus wrote:Which epic spells are worth getting (if any)?
Vampiric Feast remains the number 1 spell, but it's considered Evil so not sure if it fits your concept. Hellball is pretty nice because it ignores Evasion, Damnation is nice when folks aren't protected by PfE / PfG or Mindblank (auto-dazed, no save). Greater Ruin does 50d6 positive energy to single target so it's ok. I would say Epic spells are low priority for a gish though, so only take one if you end up with one open feat and nothing to spend it on.
Verus wrote:Is picking a specialization recommended for this theme? I generally don't find extra slots quite as pivotal for gish types, but I haven't extensively looked over the BGTSCC spell changes and additions.
This is very dependent on the player. I find you can never have enough spells. As Deathgrowl says, wizards are most powerful before they cast a single spell. Which is basically never for a gish since you are buffing as soon as you finish resting. If you want to do shapechange, SF: Transmutation will add 1 to your AC when in form. GSF = +2 and ESF = +4. For a gish I think there are more important feats rather than spell focus. As to chosing a specialty/opposition school that is up to you. You can get by without access to any given school of magic, but as a gish I would only be willing to give up either divination or necromancy. Other schools have too important buff spells or clouds.
Verus wrote:How important is AC when it comes to PvE environments? Should I be going for Armor Skin and/or Improved Combat Expetise? Or perhaps classes that give stacking AC types; or will I be comfortable in mithral full plate and a shield?
As a Gish, wearing Mithral Plate and heavy shield Your AC is 10 + 11 (MFP, 16 DEX) + 6 (IMA) + 5 (Shadow Shield) + 4 (Shield/Deez/or Shades) + 4 (Dodge boots) + 4 (Deflection item) + 2 (Heavy Shield) + 3 (Tumble) = 49, can go to 51 with Tower Shield at expense of AB, not to mention haste, mirror images and displacement. Thus, you can largely skip feats. Even if you only get +3 Dodge boots and deflection item, nothing else is gear dependent besides Mithril Full Plate and even with AC ~47 instead of 49 your mirrors will last nearly forever with displacement up. Just remember when Displacement is fighting with Haste for (extended or regular) spell slots that Displacement is more important if you are going to a dangerous area and concerned about if it is too strong for you. Haste is convenience.