First of all, wow.
Second, wow.
Thirdly, wow.
Awesome.
Like, really awesome mechanic.
Okay, the cards:
Expedition:
I can't even... Uh, hmm. Turn 3, mediocre hand, make sure stuff doesn't miss shuffle? Probably early you would rather be buying stuff, but maybe you do it if there is some trasher you want to get to a lot... Hmm, usually you probably prefer a second copy first. Maybe with Chapel. Anyway, you are mostly using it later. Maybe you can build an engine out of all this, but it seems like a pain - that is a lot of money and buys. So mostly you're... using it in a Big Money deck, as it's a lot better than silver on those late buys on dud hands? And of course just anywhere you have the spare money and buy, it won't be bad. Probably not enough to build a deck/strategy around, but a nice role-player. Interesting, difficult decisions.
TradeKevin:
This would be totally broken at 3. Most likely just too strong at 4. I feel like it's going to be pretty weak at 5. I mean, the issue is, you have to have the cards in your hand. Maybe it is good for terminal-draw BM? Yes, probably - turning 2 estates into silvers is going to be better than gold pretty much every time. An estate and a copper into silvers has to be as well. Two coppers? Well, now you need to have $7 (you need to hold the coppers back), which is asking a lot. Better than gold in a BM deck.... mmm, probably not. How about in engines? Sure, if you can't otherwise trash, or if you have cards that have outlived their usefulness (trashers say), turning them into silvers is a pretty sleek way to upgrade (not the card necessarily) your economy. So I guess this is reasonably solid anyway. Another role player, probably better for Big Money than engines, because in engines you'll so often want a 5-cost instead. Yeah.
Mission:
This one has, I think, the most potential of any card we've seen so far in the set to be broken. It probably isn't though, at least normally. Attacks that stack, this is brutal. If you just want to trash on your next turn anyway, here's your guy. And of course, it's going to be bonkers in engines which gain rather than buy - we're looking at you Horn Of Plenty. Still, I think the strongest thing will be with Ambassador, where you just need to spam this to try to win the war, yeah? I guess on your 'average' board, it hardly does anything - I guess you can skip a hand you know is going to be bad if there's no combo, but that hardly seems good. Normally weak, but lots of opportunities to be really really strong.