You will only need an implement if you decide to choose powers with the 'implement' keyword in them. Otherwise you can happily ignore them and just choose powers with the 'weapon' keyword in them instead.And for this build, I won't really have a need for a holy symbol, will I?
This is why I recommend getting a month subscription to DDI. A month is only $10 so if you don't like it, you haven't lost anything too substantial. Having access to the Character Builder and the Compendium are your best bet to answer these questions. The reason for this is because you're really getting into matters of personalisation and personal preference. I can't answer those. You need to decide for yourself what you want your character to be and how to proceed.Also, is it worth my time to get Armor Proficiency: Plate? I realize I'd have to raise my STR to 15 and already have scale... but I could retrain the AP: Scale to get AP: Plate, correct? Then just take armor finesse and heavy armor agility?
Finally, should I invest (eventually) in shield finesse?
The suggestions I've given should only be considered exactly that; suggestions. Ultimately there is no right or wrong way to build a character. As long as you're enjoying playing it, you've made the right choices.