Off the top of my head, there aren't anything wrong with that, but since you're asking the question this way I guess you can shed some light on the situation your self. Any experience with this?
My take on it would be that the purchase lead time will generate a planned purchase order as soon as the sales order is registered (assuming you use master planning and you don't have any stock or confirmed receipts within the timeframe).
If I remember correctly ATP don't take planned orders into account, so confirming the order and placing it will give you a receipt date. If you do it the same day, you won't see the order in your ATP before the next day... That is all. You won't be able to do an order confirmation before the next day if you immediately place the purchase order.
If you want the planned orders to show, you would need to run a CTP.