If all you want is a dog to bark an alert, a ton of breeds could fit the bill. A lot of the small and toy breed dogs seem more vocal to me and can be great watch dogs. Summer is the best watch dog I've had. Any noise, especially at night, and she'll start with a low growl and come over to me to let me know she thinks something's there. The few times my dad has decided to open my door and walk in at night, she literally jumps on me and barks and growls a warning to him. The only problem is when Summer barks an alarm it kind of sounds like a squeaky toy going off, lol! Most the paps have been the same way (except Nard who doesn't usually realize something until the others have, lol!) Rose is what I call my little redneck dog- she often sounds like she's going to kill whatever wanders in our yard.
Trey in his prime was a great watchdog. Shelties are very loud with their barks too so if you couldn't see him you'd have no idea it was a small dog barking at you. He sounds like he's about 60- 70 lbs and mean when he's carrying on. All of my shelties were good about this too, they tend not to care for strangers and are very observant and vocal dogs. And like I said, their bark could probably bust the person's ear drums. The only problem is they tend to not like to stop barking once they start.
Now, if someone actually came in the house, he'd probably have hidden and the paps are sure not going to do much to stop someone. (As soon as they see the person it's 'omg newbestfriend!') But I do know everything that goes on with them around.
I would suggest maybe a GSD, but they're hard (basically impossible here) to rent with. Shack was a very very protective dog and pretty imposing looking if you didn't know him. I always felt safe with him around. I feel for you on the rental thing, though. I want another GSD more than anything (yes, even a bc) but can't get one because of where I live. It sucks.
An Aussie would be another idea. A lot of Aussies are similar to shelties in that they are not going to be warming right up to strangers but they have more size. Herding breeds in general would be something to look at, imo.