Lookin at ur setup, i would reckon tat u upgrade some of ur equipment first before attemptin sps. Priority would be skimmer, wave maker and possibly chiller (if unable to cope with the new heat frm MH). Nxt, will be the CR which will help u in maintainin a constant Ca and dkh.
Personally, i felt more LR would be essential coz 20kg kinda of too little.
Followin tat...some of ur LS hav to go too =>Carpet anemone, Purple tip anemone and Anemone crab.
To answer ur queries....y nt go for 2x150W since ur depth is only 18inch coz if u hav oreadi deployed a DSB of 4-6inch....tat would leave u with a depth of 12-14inch and for this depth, 150W MH would be good enough. But of coz if u wanna go ahd with 250W, by all means.
In the scenario tat ur MH bulb blows....get it change asap. IMHO, its not healthy to deprive ur sps of light for more than 3 days and above else they will starts losin their colors.