Hello. No one has experience in estimating unknown work. There is plenty of effort estimation methodologies and they all are not exactly correct. You can use one of the methodologies used with Agile. Like the one when the team gets together and each member has a card where he/she writes her estimate and puts card face down. Then when everyone is ready cards are opened and members with biggest and smallest estimates explain their numbers and team discuss. Then you repeat the cycle with cards, and repeat it until all numbers are the same or very close. There are other methodologies, but this one is very popular. You can Google for "agile effort estimation methodologies" or like this.