A horizontal spread has the same pnl graph as a straddle except the losses flatten out at the extreme. You might as well consider it unlimited loss because by then..the spread has blown out. Don't do options without any PnL and volatility modeling.
For Excel, Hoadley is well regarded and only $100. Optionstar is Excel option no. 2. Free data connectivity for any Excel based software can be a minefield, so demo it first. Next up the food chain is web based Quickstrike through the CME. Really good stuff.
You should shop around IB's and FCM's and see who can give you the platform+analytics within your budget. Everything is negotiable if you do enough trades.
Perhaps, some nice person will demonstrate IBKR spread modeling functions.