I would go with the the most efficient miners, rather than saving a few bucks on older technology. This will keep you in the game longer when difficulty increases.
I totally disagree with that. If you are running a business, depreciation of capital assets accelerates as obsolescence increases.
There is no comparison of $120 for 1th at 1w/gh vs $380-400 for an s5. X150, This is not a few bucks.
you can buy the old hardware, hammer it out for a few months, then depreciate it on your taxes and move on to less older hardware.
in this year there are a few things to consider though.
best mining times where between 10/2014-now. Meaning one should have started early october 2014. during that time, the difficulty did not skyrocket. used cheap equipment would have paid for themselves months ago. one would be in profit at this point. in 2015, there will be some difficult factors. by may, there will be a huge difficulty jump as more mining farms get first dibs on HW with 0.2w/gh or less. so again the difficulty skyrockets. Then you will need to almost double your computing power by 2016 in order to prepare for the halving.
so can you make money now? yes, but you better start like in 2-3 weeks. then you should run all the way until you break even, then pull the plug. Recycle what you can then in sept-oct buy summer released miners.
or you can do what others do. Buy new miners, mine them for 1-3 months then sell them at a slight discount vs msrp with free shipping.
But that can complicate things on your taxes/accounting.