I would first get quotes from a couple different alarm companies. Honestly, I never get to see the prices of the installs (guess if I knew how much the company was making off the job I would be disgruntled when I saw my paystub) so can’t help you there. But really depends on the scope of work. Are they going to be pulling all new wire? What system are the putting in, addressable or conventional? How many devices? Are they just replacing the panel or all devices? Saying they need to update to 24 volts is true but shouldn’t effect the price any, unless they have to switch out devices - some can operate at 12 or 24 volts but then some can’t. As far as the time frame of the plan review, really depends on the jurisdiction. You could ask that when you get your other quotes.