[LONGHAIR]

CFM and Duty-Cycle are the most important factors to consider when looking at compressors. Many of the big box store type units are lacking in both due to cost cutting measures. They have less efficient pumps and cheaper motors, but for the average joe-homeowner, they are "good enough", to some degree.
Most guys don't use them very often, but they over-use them when they do. This results in a machine that gets abused infrequently enough to survive a while.
The lower Duty-Cycle is the worst part, some are as low as 20%. Then they put larger tanks on them to seem like you are getting something. In reality, at 20%, you should only be running for 12 minutes per hour. With a cheap compressor, having a low end pump, that is basically filling the tank (from empty) and then not running again for an hour? Not likely.
Some guys go all crazy with adding extra tanks to increase volume, but this is hard on things too. It make the pump run a lot longer when it is cycled on. This creates a lot of heat, which causes condensation as the air cools again, also not good.

A smaller unit, with a better pump and higher Duty Cycle, may be a better buy. It will run more often, but it isn't hurting itself in the process.

All of this depends on how often you use it and how long each time. Blasting and painting are big air hogs, so plan accordingly.

Air lines are another big topic.
Some guys don't like iron because of the rust factor, but there are ways to deal with that, and in some areas of the country, it's a non-issue anyway. It is a little more difficult to add or re-arrange drops, if you need to.

Copper is crazy expensive, but it's fantastic. The place where I worked as a mechanic, back in the 80s, had it everywhere, never an issue of any kind.

PVC, is cheap and it does work....for a while. When I first started at the shop where I work now, they had PVC. It was fine, until that "one time" when someone hit it. When PVC fails, it is catastrophic. This can be dangerous.

Where we are now is all aluminum. To me, it's the best of all worlds. It's not cheap, but not horrible either, it is very easy to deal with and doesn't have the safety issues of the cheaper alternatives.

One piece of advice, if you intend to have more than one drop and ever have the possibility of more than one person using air at one time, run your main line as a loop. This prevents pressure drops when someone else uses the air at the same time.