Hi Gary
I have been using a Gas/No Gas dual mig to repair  and rebuild my minis (and
others) for about 12 years. If you can work in a garage etc the gas is
preferable, ( I blow less holes! - but I am only a self-taught amatuer) but
if your outside the gasless is brilliant - no need to woorry about the wind!
Consumable use
Gasless- you get less wire and less weld per Kg  with gasless and its more
expensive.
Gas - wire is cheaper and you get more weld per Kg but you have to buy gas.

If you have a good Welding Suppliers nearby, or one you use with work you
maybe be able to get SEALEY refillable bottles - slightly more expensive to
buy but exchange for a refill.

If you have a good relationship with the gas company your work uses maybe
you can rent a small bottle for a minimal sum - you will need a differant
valve tap for this though!

Ideally I would suggest you buy a dual gas/nogas and use it in the way that
best suits the job.
Good luck
dave