There is a problem though and that's you will be taking on a lot of work since you will have 2, if not 3 roles: lead developer, Agile coach and, maybe, product owner. I've been in a similar position and it becomes very difficult to do everything. It will be essential to have your boss or the product's sponsor in the role of product owner and have the committed fully to that role. Otherwise, the likelihood of failure is going to be high and you will be the one blamed for it.
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I agree Agile/Scrum is going to be the easiest and best way to handle this.
We use Planning Poker to estimate our user stories. This makes estimation a team effort.
Mountain Goat Software is another good Agile resource.
There is a problem though and that's you will be taking on a lot of work since you will have 2, if not 3 roles: lead developer, Agile coach and, maybe, product owner. I've been in a similar position and it becomes very difficult to do everything. It will be essential to have your boss or the product's sponsor in the role of product owner and have the committed fully to that role. Otherwise, the likelihood of failure is going to be high and you will be the one blamed for it.