Using Ride115x on a large lake

I am looking to get into kayaking and found a Ride115x at a great price but I am unsure if it would be the right kayak for the type of water I am looking to paddle on. I am looking to paddle on a Rice Lake in Peterborough Ontario or

should I save up and be looking at the Ride 135?

in general
In general, for flat water paddling it is suggested to get the longest kayak in that class that fits within your budget, ability to store, and ability to transport. Length hurts off the water, but is a benefit on the water. In your case, you are looking at SOT fishing kayaks. getting up to the 13-14 foot (about as long as they go) would help with speed on the water, be a little less wide (so easier to paddle - many SOTs can be too wide to comfortably paddle), and more likely to track straight.

That said, either Ride kayak should do you fine.