Anthony,
I trailer quite a bit with an enclosed 30' trailer. Usually I am moving my '54 Imperial or the '60 Cadillac Brougham which weighs in at over 5800 lbs. Your 3/4 ton Suburban will do the job nicely, but some of the older Suburbans (including a '94 that I used to have) do not have the strongest brakes when stopping the extra weight. Just remember to always leave plenty of space in front and be sure trailer brakes are on both axles and are kept maintained. The other concern is weight distribution which is critical when trying to keep a trailer following straight. I think you should consider a larger trailer to allow forward/rear movement to properly balance the weight. You need 10-15% of total weight (trailer loaded with car,etc.) on the tongue of the trailer. If you will always be towing the same car and contents, you can set it up properly once and add weight to ffront or rear of trailer to properly distribute things. Tie down is also important as you will always want car tied in same place as tongue weight changes as car move forward or back. The 18' trailer will not allow much movement to distribute weight, so I would load it and check tongue weight before committting to verify that you can get proper weight distribution.
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