No definite start date, but I project it'll begin sometime next week to the week after next (if you include the slow story progression that always comes before the BD part).
125 S/D is suggested because the next S/D boost (at 200) is the biggest jump. After 200, it's every 50 stat points.
If time and NP is not a concern, you should go to the highest boost you can, that you can get to while the plot is still active. Otherwise, 125 is a good place to stop.
The above weapon suggestions are great for price-power ratio for the occasional or poor BDer, hence they will likely go up in price as the war unfolds, as everyone will be buying them up for use.
Your first buys should be the 2 constants. Go for the 11-iconers first. The other ones are only important once we know what the BD challengers' defence weapons are, should what weapons you have, be blocked often. If you can afford the non-SR dual duties, do them instead. Physical will be in 90% of battles, so RP is a big plus.
Then you should get a reliable healer as 2nd. Since autoheal after winning a battle is now gone, it's either heal in-battle, keep buying potions outside, or random Springs every half an hour. GHS and Blue SS are the ones to aim for unless you can afford better.
GHS doesn't overheal, while Blue SS does (eg. if you healed at max HP, you'd be +30 over full HP). In fact, any SS outside of Jade and Ice overheals.