Regarding ceiling clearance, I had a contractor put a laminated beam across the garage to new posts at each side, sister on rafters to the top of the trusses, then remove the truss structure. Solved the head room problem.
You definately have to pay attention to the overhead door clearance. If you have a wide door and you center the lift, there's no problem when the door is closed. Be careful when it's open....
I have a 4 post 12,000lb unit (2 post units make me a little nervous, but that's just me). Makes it REAL EASY to change the
oil and clean the undercarrage......Let alone everything else you'll need to access under a car/truck/etc.
Access is good. To deal with tires, etc. some companies make a bridge to go between the rails for bottle jacks. I would recommend this with the lift.
If you are going to work on cars, I highly recommend the purchase.
I installed mine myself. However, if you do not have equipment to handle very heavy things (800-1000lb +-) you might consider hiring a professional to do the install. You will also need 4" of uncracked concrete in the floor, to bolt the thing to.
I bought a Bend-Pak unit out of Northern Tool. I like it just fine.