Yes. Sweet potatoes and white potatoes are in two entirely different families. White potatoes are a nightshade, while sweet potatoes are part of the morning glory family. White potatoes are closer to a tomato than a sweet potato. If by "yellow potato" you mean a "white sweet potato," they are entirely different.
You want a cheap bag of russet potatoes. They're usually about $2/5 lbs
That sounds okay. I assume you are not counting carbohydrate from fibrous vegetables? I wouldn't count carbs from any greens or vegetation. The lowest-carb carbs I'd count are squashes, if that makes sense.
Try experimenting with different carb sources first. I have my preferential starch list and I go as follows:
Sweet potatoes, white potatoes, plantains, white rice, nixtamilized non-GMO corn tortillas (they MUST be non-GMO and nixtamalized, meaning they're "treated with lime (calcluim carbonate, not lime juice)" which makes them a clean starch). Sweet potatoes are my favorite based on taste, but bloat me the worst. White potatoes and white rice bloat me the least. Try them I'd say.