No, the 18 won't be enough. I have an OXY 30#, and have tried a 18" Oxy (hot pink! It was awesome) a few times.You are about 20 lbs more lean than I am, and in freshwater, the 18 was just barely enough buoyancy to to keep my lips out of the water. That was in a 3mm two piece suit. In salt water it would probably be just enough, but only in calm conditions. The difference in size/drag between the two underwater is insignificant, just get the 30.
With the 30, I have dove everything from no suit tropical diving to 5mm wetsuit to drysuit. Al63s, AL 80, steel HP100's... doesn't matter.
For back plates, I'd go with aluminum or dive rites travel ss plate with all the cut outs to keep the weight down. If you find a plate/wing combination that doesn't work together, I'd be surprised since they seem to be pretty universal.
-Chris