I use IB not commsec, but using a buy limit order like you said above the previous close will allow you to get filled in the opening auction, where a MKT order will only get filled once price has traded. So a LMT order will probably be better if you are after a fill on the open. I use 10% to ensure i get filled and IB doesn't have any issues, that is for a sell though i use it for.
It sounds strange that you can't place a stop in the market before you take the trade. If you want to trade EOD and you have to be available during market hours that sounds a bit counter intuitive. If commsec can't provide the facility you might need to change brokers? Plus Commsec comms will drag on your account if you are trading a fair bit ( I believe they are fairly high).