Make the timeline as short as you can to guarantee that everyone is working towards the same timeframe.
The more control each should have of their part of the deal the better, as you’ll receive far better financial results if you have multiple partners working towards the same goals.
When talking with lenders, always present a well-structured plan and have a plan B in case one of the others doesn’t execute on the purchase. The lender is obviously going to want to confirm your numbers, but they may also want to see an estimate of the estimated timeframe, a copy of the sales contract, a copy of the deposit agreement, the current market, etc. Nobody likes a hinderance to closing.