Guessing you want this to be your project or as mentioned you could just buy one.
Here are a few safety issues to think about:
1) Stability, either a wide base or bolted to the floor.
2) Plenty of overhead room.
3) Access ramp to get the bike on the ramp. Don't expect to drive it on or you may end up on America's Funniest Videos.
4) Rugged fixture for the bike, not only to keep the bike in position but allow you to push and pull while working on it.
5) Positive position lock, assume it will collapse. But not so accessible as to be disengaged by accident.
6) Factor of safety of at least 2X of max load (bike, you and your future bike) if your bike weighs 2000lbs build for 4000 lbs.
7) Positive safety lock, when not in use no one else can activate it.
8) Simple control design, if you need help you don't want to have to explain it or mistake up for down.
9) Controls and locking device accessible from a safe distance.
10) Emergency shutoff, if powered.
11) Travel limit indicators, so that you do not exceed the safe range of operation.
12) Pressure indicator on the bottom face, to prevent crushing.
