the time for break even or roi, will not change based on your investments, so if you invest in one antminer s5, it will be the same if you invest in 10 antminer s5...
initial cost/ total hash power ratio will always be the same
Unless you can get a better deal by buying in bulk, or for example, buy a PSU that can power several antminers. You'll need to buy a PSU wether you have 1 or 3. (i guess this goes for all infrastructure costs: they'll become cheaper the more you invest... Cooling, power, storage, security)
Also, if you set up a big farm and register as a company, you might get you power cheaper?
I'm just a beginner myself, so i hope these assumptions are correct?
yes i presume that if you really want to go big(but the risk is also higher) you can find some promotion, that will help you paying less, thus making roi less painful, but i don't expect the difference to be that huge
the cost of your electricity depend on wwhere you live for the most part, registering undersomeone may help, but no so much, i know that china and washington offer a very cheap electricity, below 0.05