Sounds like whoever made the offer is playing the Cash Flow game. Long (ish) settlement, minimal deposit so there cash sits in an offset account for 90 days, rather than in your agents T/A. I'd do the same myself.
There is no real hard and fast rule on how large.small a deposit needs to be. If you're happy with 5%, then go for it. If you want 10%, then push for that.
If they are going to back out of the deal down the track, how 5% or 10% won't really factor into it, I wouldn't think.
If you're happy with the offer and the conditions, then go for the win-win solution and take the 5% deposit. That's what I'd do, anyway.
So, is this an offer from the buyer who made the intial offer or has another come out of the woodwork?
The auction isn't going ahead, then?
Good luck!!!