200 Amp Service To Detached Garage (followup)

CR295

The basics:
Running 200 amp to a detached garage from a 200a disconnect beside meter. Its a 90' run. I think that I'm going with 4/0 aluminum URD in conduit.

You guys pointed me in the right direction earlier by recommending 4 wire (there are presently no conductive paths connecting the two structures) but may be in the future...ie cable, phone line, etc.

So my question is what do I ask for when I go in Rexel? I actually went in yesterday and the guy behind the counter didnt appear to know exactly what I was looking for. He didnt know what URD meant. Anyway he showed me a 4/0, 4/0, 4/0, 2/0, service cable I forgot to look at the designation, ie TW, THW, THWN, XHHW...he said it would not work direct burial but would be fine buried in a conduit...not that I trust him at this point! I'm assuming this would be two hot, one neutral at 4/0 and the ground is 2/0? All of them appeared to be individually insulated and then contained in a gray outer sheath. Is this in the ballpark of what I'm looking for?

It was also recommended on this forum that I could use "mobile home feeder cable" He had no clue what that was...he said the RV dealer is down the street-LOL! I think between the two of us our ignorance was contagious! We both walked away scratching our head.

My next question...the area I'm in has a high water table...basically a beach area. You go down 2-2.5' and your trench starts collapsing from the water. The upside is the first two feet are easy to dig-sand. Are there any special consideration I need to think about in running the conduit? Would direct burial cable be out of the question? Would direct burial cable inside conduit offer any more protection? I'm wondering if this "WATER" condition would qualify as "subject to physical damage?" I'm guessing schedule 40 conduit would be ok...thats whats run from the pole to the house...doesnt mean its right though?

My last question...I'm planning on running the trench, conduit, and wire and leaving the final hookup to a pro. I noticed this 4/0 feed is almost as big as my wrist. Can I snake it thru the conduit as I go (while its laying beside the trench) and then glue up and drop in the trench? If so I think 2" conduit will work. How much extra on each side should I allow for the hook-up when ordering the wire? Sorry for my ignorance and all the questions. Thanks in advance for your advice!