Just to itself, mismatching tires is not "unsafe." But it is not ideal, and under the right conditions it could be less safe.Why people are so often willing to be cheap with tires can be very confusing! Ultimately, tires are the one element that effects everything other aspect of your vehicle. Nothing else touches the road. How well your car accelerates, brakes, manuevers, handles going through water, ect, is all limited by the ability of your tires!Mismatched tires mean you have differing levels of grip between the different corners of the vehicle. Under normal highway driving where the tires are not being pressed to their limits, this could merely mean one makes more noise than the other or rides rougher. This all changes the moment you must make an emergecy stop or manuever to avoid an accident, and you are then at the mercy of the weakest link in the chain (moreso if it's wet or traction is otherwise limited).Also, the recommended pressure on the tires is based on the vehicle, not the tires themself (the value listed on the tire is the maximum safe pressure, not the recommended pressure). Check on the inside of the driver's door, the tag there should list the recommended tire pressure.Any set of basic touring tires should work fine (all the tires already mention would probably work fine)... Tirerack.com is a good source for tire reviews if you want to compare to specific tires.Tire cost can vary quite a bit depending on what you want (high performance tires cost more, and do not last as long). But $40-80 per tire is probably a reasonable range for the size you mention, with another $15-20 on top of that for balancing and install and disposal of the old tires.