Only 1 trade PER EVENT?
Use the "No Trade" block. If you have no trade, then your conditions go off for a BUY, it buys until it closes it out or you close it out yourself. If you use the default "No Trade" block, it will prevent any sells from opening as well.
This block will also pick up manually open trades. If you wish to trade manually as well, you'll need to great a GROUP for the ea