Two things.....One, you don't need a smith machine to be safe. The only potentially dangerous exercise is bench. But if you buy a POWER CAGE, you will be fine. If you get one with narrow gaps between the hole spacing, you will be able to set up your bench within the cage. The safety pins will be in such a spot that if you can still do a complete rep. Should you fail however, the pins will be slightly lower that your chest yet higher than your neck. Thus, if you are failing, you can let the weight down "on your neck", and the pins will save you.
There is a thread about newyorkbarbell that was done just recently. Get a decent power rack from them and the above will be solved. And for cheaper than your smith machine. I can even link you to the one I have if you decide to take my advice.