RDD is generally a fighter class. Most RDD people take bard for one level just to start RDD and then continue down the fighter line. The only really good RDD/sorc builds IMO include the MotB expansion as a neccessity to make level 30 chars. That way you can do 9 sorc/10RDD/10EK/1 fighter so that you can later take automatic still spells and wear full armor without worrying about arcane failure. Unless you get the EK armor, that is, but either way you need to become EK.
Overall EK/sorc is a better melee caster build than RDD/sorc. at lest you don't lose your spells along the way. The + stat bonus really doesn't compare to the loss of 10 spell levels.
Rage stacks with the bonuses from items, but because the maximum temporary increase you can give to an ability score is hard coded to 12, Rage becomes kind of useless in Mask of the Betrayer when you have a belt that already adds 10, or even the belt that adds the full 12 itself (although that one applies some penalties to other stuff). In the OC, you can still make a +8 STR belt, which means that any version of rage more powerful than the basic one will still be wasted.
Oh, and back on topic: Red Dragon Disciple is not meant for sorcerers, even though they unlock it. Bards can get some use, but again, it isn't really a good idea. Take only a single level of either class (and it should be bard unless your build requires lawful alignment) to unlock it and then treat RDD as the melee-oriented class it is.
as far as weapons, i'd say greatsword or greataxe. some big two handed weapon to take advantage of the 1.5x strength bonus to your super high strength stat. armor depends on what you multiclass with and what's made available to you.
i have a question though. i'm making a bard/figter/rdd build. taking it up to lvl 20 (don't expect to get higher in the OC). don't want exp penalties for multiclassing. using a human. don't care about spellcasting, so expected to take fighter first. extra feats, skimp on cha and put into str., but i'm curious, how should i progress from there? i know i gotta keep the fighter and bard levels one apart to avoid the penalty, but i'd rather not take the bard levels at all. don't care if i multiclass with a barbarian as a fourth, but i would stil get the penalty, if not a bigger one for having so many classes apart.
... Know ye what a favored class is? It's a class that is completely and utterly ignored for multiclass XP penalties. All prestige classes and the favored class are completely ignored. In the case of humans, their favored class can be anything. It's their highest non-prestige class. A human Druid17/Fighter1/Monk1/Rogue1 gets no XP penalty because Druid is ignored and the rest are one.