"My biggest concern is running the loop overhead in the attic. Will I be ok doing this?"

As long as you make SURE the line is on the warm side of the insulation you should be OK.

Insulation does NOT stop movement of heat, just slows it down.

"Also, would appreciate any input on electrical systems to trigger pump in response to motion sensors in baths and kitchen during times of use/demand."

Look for a motion sensor with a form A or C relay output.
Use a 24 VAC transformer designed for thermostats.
The low voltage and power allow you to use MUCH simpler wiring methods.

You can then run thermostat wire to the transformer & pump location.

Use a common relay to turn the circulating pump on and off.

By using typical parts designed for HVAC you will save a LOT of money.

Temtrol DeltaT makes a TLC-X1 (thermal logic controller) which you plug any pump into and it will cycle the pump based on user chosen temperature using multiple types of activation methods. Occupancy sensor, Wireless, push-button, timer, etc. It allows adjustment of the shut-off temperature setpoint by the homeowner to their desired temperature. Temtrol makes Redytemp hwc systems utilize Taco 00 cartridge circulator pumps. But you would only the the controller TLC-X1

I would double check your flow requirements to ensure that whatever pump you use will deliver the hot water in an acceptable time period.

Here is a link that might be useful: hot water circulators and controllers

Looks at least a little overpriced at $150.

>Looks at least a little overpriced at $150.

Do you have another model suggestion? Plus any other components required.

I've just started a major renovation, and am wondering what the cost of adding a demand/motion sensor recirc loop would be, so I can decide whether it is worth it.

I'll have about 65' of return run.

It depends on what yo9u want the controller to do.

If you want a single package solution buy it.

I usually avoid that sort of thing since if any part breaks you have to replace the whole thing.

A separate timer, check valve, thermostat type transformer and relays should be less than $150.