Use a limit order for your 1 lot and you will not have any slippage.
You may not get filled but this is the trade-off you are making, the trade will take place at the limit price only.

Slippage won't be your real issue trying to develop a system like this.

The 3 major issues are:

1 - Getting filled on your entries, or rather having the system report realistic fills
2 - Optimizing the system to get it to work on historical data which leads to failure in real time
3 - Trying to program something you don't know how to do. So if you cannot currently trade, it's unlikely that you will be able to code a trading system