Yes you can make roast potatoes ahead of time and then reheat them the next day in the oven.
Although roast potatoes are best when made and served the same day without reheating them you can still reheat the roast potatoes if you really need too.
Just place the roast potatoes into the refrigerator overnight and cover them with some plastic wrap or a lid on a container with the roast potatoes.
Then just place the roast potatoes into a preheated 400 F oven for 10 or 15 minutes to heat the roast potatoes back up.
But if you want the best tasting roast potatoes it's best to prepare them and cook them the same day.
Both sunflower oil and olive oil are good cooking oils for making roast potatoes.
I personally prefer to use olive oil when making roast potatoes but the sunflower oil makes good roast potatoes as well.
Depending on how many roast potatoes you're making it can take between 30 minutes to as long as 2 hours to make roast potatoes.
You can also use any type of vegetable or cooking oil to make the roast potatoes with and they will turn out just fine.
But I prefer the olive oil for roast potatoes as well as making french fries or just frying potatoes in general.