As a fairly long time hybrid owner (Ford Escape, but uses technology under license from Toyota) I will weigh in. Between me and my son we've put 80,000 miles on this vehicle for a total mileage of 140K+. Being a DIYer I've done most of the repair work needed, mostly after 100K. As far as the hybrid system goes, the motor and battery themselves are designed to last the life of the car and are not a concern. The vehicle's electronic controller strives to keep the battery at around 40-60% state of charge, eliminating the wide cycles that kill batteries. Some people will "tune" the car to allow a deeper discharge to improve mileage, but this comes at the expense of battery life. The issues that I have had are with the systems that manage the temperature of the battery, engine and transaxle. All of those systems failed within about a 20K period, and I replaced all of them (thermostat,battery cooling fans and transaxle water pump) myself at a cost of less than $400. Dealer cost would have been $2000+.
Having said all that, I would not use a hybrid for towing, since towing will stress all of the hybrid component heat management systems much more than regular driving. Also, while I'm not sure about the Highlander, the Escape Hybrid used electric mode only for reverse, which severely limits the amount of power available to back up. This could cause big trouble if you're trying to push a trailer up hill.
If you could find a good deal on a used Highlander hybrid it might be a good purchase, since the first owner has eaten most of the hybrid premium. However for the reasons I mentioned I would not tow anything more than a small popup with one.