You could start with a 3 month KTA with the wording that either party can cancel the agreement at any time and that they pay 100% upfront. If they end up being a PITA, refund them a prorated $ amount and wish them good luck. If they want to cancel (because that probably means you're doing a crappy job) then they deserve to get their money back.
The short-term KTA also lets you get to know them better and you'll be in a much better position to price a long-term KTA.
If they're off-plan, they are telling you upfront that they are very price sensitive. In a way, they're doing you a favor.