When you have a mining barge (when you get into the exhumer that is) you'll still only have 1/2 to 1/3 of the cargo of an industrial. If you have someone in an Orca (the mining industrial) it'll hole 100 times what your mining barge will hold. You'll be moving back and forth to station a LOT more than you'll actually be sitting still mining. Mining with one ship = not the way to make money. You'll be in transit (where you aren't making isk) a lot more than you think
In terms of what ship to buy 'to fight' well. If you are doing PvE, work your way up the ladder through cruiser to Battlecruiser. Stay there until you can fit it properly (adequate tank and proper sized guns). No sense in training into a Battleship that you can only fit medium guns or missiles on because you didn't train for the weapons.
There's no "right" or "wrong" way to go about things in eve. Just an easier path, and a harder path. An argument that I hear a lot is "This is EVE you can play however you want" which is true. However if you are never making money and always losing ships/flying around buying things at "lower" prices while you fail at managing oppurtunity costs, it'll take a while to make any money. However, all the while your skillpoints will (hopefully) be racking up