Set download limit to 1GB for free users and upload 999MB filesize. If you don't force users to buy premium, they will do it by themselves, because no one likes to download 10GB for 3 days
My experience tells me that they buy premiums even on 40-50MB files, when they want. Else, they will never download or look at your post again if you force them.
Those who don't want to buy, they will not buy. You simply CAN'T force them. But, if they downloaded for free from you, eventually, they will buy it, because they will hate time waiting, and captcha. Most important is that you keep free users to yourself, and one day, if they want they will convert.
And ofc, no mirrors