I'm working on an assassin for a PbP, and I want to make sure I'm doing it right.
The way I read it, some assassin powers have the implement keyword and others have the weapon keyword. This leads me to the conclusion that those with the implement keyword use any enhancement bonuses from my magical ki focus and any feats that improve powers using that implement (such as implement expertise). Powers that have the weapon keyword would use the enhancement bonuses of a magic weapon and of any feats that improve my ability to use that weapon (weapon expertise, weapon focus, etc).
Is that right, or am I missing something?
Assassin's actually can work a bit more interestingly than that. They can use their Ki Focus to replace the enhancement bonus of their weapon - or they can use a Weapon as an Implement.
With Ki Focus, you can use it normally as an implement, of course, adding its enhancement bonus and any feats that improve powers using that implement.
When using a weapon, though, you can use it's enhancement bonus when making a weapon attack with a weapon the assassin is proficient with. If you have both a magic ki focus and a magic weapon, you have to choose before the attack which magic you are using. For example, if you have a +3 Ki Focus and a +2 Flaming Dagger, you can make your weapon attacks with either a +3 enhancement bonus, or with a +2 enhancement bonus and the flaming property, but you can't combine them into a +3 enhancement bonus and the flaming property. Note, however, that for all Weapon attacks, you will still be relying on feats specific to that weapon. (Weapon Expertise, Weapon Focus, etc.)
In addition to this, an assassin can use any weapon with which they are proficient as an implement. This functions just like other classes who use weapons as implements (Swordmages, Sorcerers). You would probably want the Focused Expertise feat in this case (which would provide the bonus both when using it as a Weapon and an Implement), and the non-magical properties of the weapon (High Crit, Versatile, etc) would not apply when using it as an implement.
In general, if you want to be really focused in a single weapon for all your attacks, getting a magic weapon and using it as your implement is the way to go, since Focused Expertise and Weapon Focus will then apply on all attacks.
Using a Ki Focus, on the other hand, is ideal for the character who wants to have a wide variety of options - to be just as capable when using implement powers, then switching to hand-to-hand combat, then disarming a guard and using their own longsword against them.
I hope my explanations here have been clear - it is a somewhat tricky and new mechanic, but one I think allows for a wide variety of options for those characters. Both the Assassin and Monk compendium entries do a decent job of explaining it, and they've cleared up the language quite a bit from the original articles.
Could an assassin use the properties of a magic melee weapon with one of their powers that contains the implement keyword and is a ranged attack?
Yes - since the assassin can use the weapon as an implement, they can use it for any attack with the implement keyword. They essentially focus their power through the weapon just like a Wizard might through a wand, for example.