Get the largest tank you can buy without the welding gas company requiring a contract. When you need it filled, drop it off and exchange it.
I would not by one at a garage sale or auction because technically they are owned by the welding supply company and if you bring it to get it filled they will say thank you for returning it.
One of your first projects can be a welding cart. It is a good place to start.
As far as cutting, what do you want to cut? Before I got a laser, I had a plasma and a torch yet I cut nearly everything with a Harbor Freight grinder and cutoff wheel.
It is a dangerous tool which requires all of the safety gear however...it is extremely capable of cutting most things with precision both heavy and light:
Did I mention this is a tool which is as dangerous as it is capable.