I believe most/all Simplisafe systems have a 24-hour battery backup. 

Some of them will reset to 12:00 and no program when the lights go out; some will handle short outages but not long ones; some of them have a battery and will keep time even with long outages. 

I offered to stop by the same day (last time the battery needed to be replaced in the thermostat) but he said it was not a good day for him.

One low-key method I like to use is to schedule a visit to change the batteries in the smoke detectors.

WiFi is very battery intensive, so I highly recommend that you use Z Wave or ZigBee.

Electrical not up to code and smoke detectors (not a big deal w/ battery install) definitely warrant some negotiation as they aren't obvious to the typical home buyer.

The safest setup would be to have hard-wired smoke and CO detectors with sealed battery backups.

Batteries, forget about it. $10K.Still, it may not work for everyone, but it's an interesting option. 

Depends on what you are doing with the house, if you are gutting the place and putting new wiring in, then you may have to have hardwired smoke detectors put in to meet current codes, if you are just painting the place then you can get away with battery operated smokes. you will need utilities hooked up and either working or up to code, depending on what you are doing.

If there is no work order for sometime and then scheduling a walk through would be fine, or even telling them you are coming in to change smoke batteries.