You should balance between HP and crit. The best PvE pizzas at the moment are BSH and mushin’s tower ones. They have random stats, so it may take a while to make them perfect. In the meantime, go for whatever combo you can make that gives you the most HP and crit. Go for whatever stat you seem to need more – if you’re comfortable dodging, get more crit over HP, opposite if you have troubles avoiding damage. If you feel like you’re really getting pounded all the time, comboing one of the two mentioned sets with moonwater for extra hp is also not the worst plan.
Stats are randomly “rolled” when you unseal a piece and can’t be changed. In other words, hunting for the perfect piece involves lots of grinding and unsealing, which may not be very cost much Blade & Soul Gold. It’s something you gradually work on.
Fusion UI works just like weapon fusion. The shield you use as base keeps its stats, but gains a random amount of secondary stats from the offering. Minimum and maximum possible gains are determined by how much stats the offering has. So for example if you offer a piece with 50 crit, it will have considerably lower range of bonus values it can produce compared to one with 200 crit. The end result is again RNG though.
Get your starter set, this can be either poh or moonwater arena or a combination of the two. Then start grinding bsh and mushin’s tower and don’t stop until you’re perfectly happy with what you have. If you mix two sets, make sure you do it as 3+5 for set bonuses.
In poh24, every man and his dog drops blackram critical pieces. The best one for fusion is number 8 (116 crit methinks), but really anything goes. This is pretty much free as you’ll pick them up while doing your Blade & Soul Items, and if you’ve been breaking useless pieces you’ll have literally metric tons of fusion powder.
Maximum bonus is determined by how much crit the offering has. You cannot stack offerings though, your bonus is rewritten every time you fuse.
3 sealing charms cost roughly 30 silver, depending on your RNG, anywhere between 2 gold and 40 silver and… A whole lot of money. I’d expect something on the order of 20(ish?) gold for a finished product. Can’t tell you for sure, my rule of thumb is “do it ’till you’re bored, then just work with what you’ve gathered”.