A simple keyed security doorknob is about all you can do. You'll need to clear it with your landlord and probably have to provide him with a key.
There are some that are pretty good - for what they are - and some that are merely fair. It pretty much goes with the price, the better ones cost more. I prefer Schlage for homeowner-level stuff, and won't install Kwikset unless someone orders me to do it - and even then, I'll argue with them.
In any case, you almost certainly don't need to call in a pro. If it only takes a few screws to remove your existing door knob, then you won't need to do anything special to replace it either.
So let's measure: if the hole through the door is 2-1/8" in diameter, and its nearest edge to the edge of the door is 1-3/8 or 1-3/4 back from the edge. If those are your dimensions, you have a standard door knob opening and what's available on the shelf of pretty much any hardware store or home center will get the job done for you, and the instructions included with it are clear enough that if you can find the pointy end of the screwdriver, you should be able to install it with no trouble.
But again, check with your landlord first.
Good luck with it.
 Unless I miss my guess, isn't the landlord required to give notice before the make entry to your home? Yes, it's their property, but if you are up on your rent and they have no reason to suspect illegal activity, you have a reasonable expectation of privacy.
You might want to consider installing a webcam on your home computer, or even just a cheap trail camera, and leave it someplace to monitor your digs. If the landlord is accessing your space without cause, you might have a solid case to lodge a complaint against him.