If you can get your materials at cost,and your wife is a HVAC tech you are set.You say you can weld and fabricate.I will give you advise if you need any,as the same as others on here will.You will find building liqiud water vessel more demanding than just making some brackets and such at a service shop.I have been welding and fabbing for 20yrs.the last 12 yrs as a journey man millwright.
I don't know what machine you have or it's range of capacity.You will be running some high amps to get good penetration during root passes on 1/4"or heavier material depending on what you go with.
Make sure you keep good ventilation when welding in the tank.Due to the heavy shielding gas.
You will also find that this baby will get heavy and unwieldy at times so make sure you have proper cribbing and rigging to support it as you are building it.Take your time and plan ahead for future improvements.
Don't set yourself on your first design you come up with.You will find different and better ideas as you go.Just add up all the features you want to incorporate and go from there.Plan out how you will assemble and weld different joints and pieces..As sometimes you might not be able to get to them after another part has been fabbed.
This will be a fun project for you.I enjoyed making mine and love knowing that it's keeping my family warm all winter.
I just got done pulling 16 hrs so if this makes no sense don't pay any attention.I think there is a bed calling me somewhere
oh and good luck and take lots of pics