Answers

If you are installing a Hot Tub you will need a Concrete base or a solid base of Pavers or Stone, it must be level to distribute weight evenly. This ensures no water damadge to the bottom of the unit as well if there is any type of warranty on your unit the base insures you are covered for certain damages. I always suggest to my clients they should read the manual and installation instructions to see why they are installed the way they are. Below I have attached some alternitive options for a base other than concrete. I hope this helps!! wood and Flagstone option below. the 220V is important because you want to know your electrical will no fail without enough support, you will need a dedicated breaker with the 220V which any Electrician with your company or the clients will be able to do this. it will make it through think and thin. 

Search for "barn movers". That may be a good place to start. Also, ask local spa companies for recommendations

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Yes you really should.
With freezing temps, frost, and the funny movement of the earth and setteling its best to just do it right the first time.

Pour a 4" slab with some 18" OC Rebar (18" on center).

Hook up the 220v with a dedicated breaker and walk away knowing that it will last for YEARS!

I'm not familiar with cartridges but you can buy an aromatherapy canister (hot tub warehouse online has them) in which you put aromatherapy beads whichare available many places (such as spa depot). You can add aromatherapy beads and liquid directly into the spa as well.

There is several different places you can purchase these. Walmart has some option, home depot, bed bath & beyong, etc. Maybe check these places online and see what they have. It would probably be very simple for you to find the best product online, otherwise try calling one of those store to ask if they have any in stock!