Those two are very close to the ones I was looking at. Mine is a PJ
http://www.pjtrailers.com/trailers/b5-5-inch-channel-buggy-hauler 20' deck, no dovetail.
After having towed about 10k miles last year on several trips here's my thoughts on trailer:
1. As far as length you're pretty close. I've done a lot of research and it seems to vary by state. I don't think any are under 65' though, and if there are its just a few. That said I get the impression this is RARELY enforced and not nearly as important as weight. I'd like to see the trooper get out the 100' tape to check if you're over by a foot or two. To be honest unless you're grossly abusing length I'd be very surprised if I had an issue. If the opportunity comes up to pull mine and a buddy's on a 32 footer now and then I wouldnt hesitate to do so.
2. Dovetails - Depending on how your jeep is setup a dovetail may mean the difference between using ramps and not. With the 37" tires on mine I can just barely not get on it without ramps. A dovetail wouldve been perfect. I'd guess you'll need a least 35's and a few inches of lift to make it work though. Either way I'd opt for a few feet of dovetail
3. Length - At 20' deck I've got 3' spare on the front of the trailer with it balanced. 18' should work for a 4 dr, but its also nice to have a few spare feet to work with for weight distribution. I put a 48" job box on the front of mine now for storing spare parts.
4. GVWR - I'd highly recommend 10k if you have a 4dr or pickup. When I weighed myself I was almost 8k loaded w/ the trailer but that was before I added the spare parts and box. I dont like worrying when I tow so not having to be towing with axles at or over their capacity is a no brainer for me.
5. Deckover - I did look at one of these and tbh behind these monsters you'll never notice the difference in height. The one I looked at had 7k axles (14k GVWR) but I'd never use that. The trailer was almost 1k lbs heavier too. For me the buggy hauler at 2700 lb was perfect because I can still use it behind a half ton every now and then if I want to. The deckover would really be pushing it.
6. Width - I'm exactly 83" wide with 14.50" wide tires on stock axles. If there's any chance you'll have wide tires or 1 ton axles you'll be over that and want a 102" wide deck. The drive over fenders on the car haulers are perfect, so still not forced into a deckover. If you're not going to do that you can probably stick with an 83" deck with removable fenders (for the oddball times you may be wide). The narrower deck just opens up way more possibilities in terms of inventory.
All in all I think you're on the right track. Just look at the axles on that first one I'd suggest. Other than that these things tow BEAUTIFULLY. Seriously barely even notice its back there.