So let's say that I pick Doppelganger. Doppelganger comes at an 8 ECL because he's 4-LA and 4-HD. Is this right?
Some minor changes have been made to the character creation rules since that first recruitment thread was put up. For your base monster, you can use either CR or HD to calculate ECL, whichever is higher, while you still have to use LA for templates unless a template that you want to use doesn't have a listed LA, at which point you use CR. So, for the CR 3 Doppelganger, you use those 4 HD for your ECL, leaving you with up to +9 LA that you could use to pick up one acquired and one inherited template.
Though, I'll note that there is also the option of the Greater Doppelganger
in Monsters of Faerun, if you'd like to be even more of a shape shifting infiltrator type. CR 12, so you'd only have room for a single +1 LA template, though.
Well, you would need approval for any homebrew classes what you use but I approve the dragoon lancer on that link. Anycase you could take half-dragon template (fits for dragoon theme and gives lots more Str) as inherited template. With that you have taken 11 monster levels. So you can take +2 LA template, like half-vampire perhaps? Bit more Str and some thematic vampire powers (being offspring of vampiric dragon who trained you to become dark knight for it? Or vampiric influences come from touch of encounter with one or result of powerful necromancy or cursed artifact).
I'd go for the Lolth-Touched template, myself, when it comes to picking up acquired template for boosting Strength. That could lead to some interesting possible roleplaying options, too, I think. ^^