I have had 2 setups that I bought used. I highly recommend it. I would go with the larger tank because the water chemistry is more stable, essentially it will take more of anything to have the same effect on the chemistry compared to a smaller set up.
Moving the system is relatively easy. You really need a few buckets, or a couple of big rubbermaid containers. I have done this before and used treated tap water , I still use treated tap water in my 100g tank, so you don't have to panic if you don't have an RO setup yet. If you move the livestock and rocks in buckets or rubbermaid containers, you will end up with a fair amount of the original water. I would suggest that you get the salinity and temp of the water before you pick up the setup so you can premix some top up water for the system once you have it set up.
The key thing is to keep testing the water at least once a day for a few days to keep an eye on ammonia and nitrates. Be prepared to do a few water changes over the first two weeks.
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