The Lawnbott LB3550 is getting a bit long in the tooth now (or would that be long in the blade?), so you can probably pick it up at a bargain price if you do a little shopping around on the Internet. It's not as quiet as some of the recent models, but it comes with all of the features you'd want. Its double lithium-ion batteries make it a good choice for cutting particularly large lawns and it comes with a Cube 3D Compass (an optional item, but usually sold with the bot anyway) that helps it find its way around particularly complicated lawns, especially those with slopes and hills that would baffle a lawnbot with a less sophisticated sense of direction. It can cover 40,000 square feet of lawn before its batteries go dry and comes with an extra set that you can swap in if you need the job done faster than the charger will let you get the first set back up into running condition. The Compass includes an accelerometer to keep the mower from becoming disoriented on slopes, so that it always cuts on a straight line, even when the terrain does its best to defeat it.
You can get an optional set of spiked wheels to provide extra traction for it in case your lawn is particularly hilly or you can just depend on the wide, notched wheels that come with it standard. However, these wheels can only handle up to 25 degree slopes, so you might want to get the optional spiked ones if you want the climbing power of some newer model lawnbots. The LB3550 uses a guide wire that you can install to divide your lawn into up to four separate zones, so the mower will only go where you want it to go and not plow down the flower bed or plunge into the swimming pool. It uses brushless blade motors that the manufacturer claims will take eight times longer to wear out than a lot of its competitors' motors.
If you're not looking for a used mower bargain and would like to try something newer, look at the Husqvarna 450X, the Robomow RS622 or the Gardena Sileno, which is purportedly the bestselling automower in Europe.
The motors do not have brushes in them so you don't have to worry about them wearing out. They work great...mows on a schedule that you set.