As mentioned, an engine seizing shouldn't affect the transmission. However, since you can't drive it there is no way to verify that there isn't an issue with the transmission until you put the work into the engine (or if there are other driveability issues), so it's a bit of a gamble.
The motor shouldn't affect the security system. As long as everything is installed correctly and the old computer and electronics are all used, it should be a "simple" swap.
Again, I would make sure you look at everything you can (steering components, diff fluid and transmission fluid and levels, exhaust, driveshaft boots, etc.) and see if there are any issues. Since you can't drive it a good look over is in order to make sure there won't be any surprises. Even if you are going to buy it either way, it may give you a bargaining chip if you find something wrong.
Edit: Also, how good are the tires? Decent tires that size can be upwards of 1000 dollars, so that's something to consider as well if they are about done (can't tell from the picture, but the spare looks worn out, no idea about the rest).
Last edited by BigTB; 09-28-2015 at 09:10 AM.