Irrespective of which type of cryptocurrency you want to mine there are some general principals when thinking about profit.
Cost of equipment: If you have to buy new equipment, then before you make a profit you need to pay that off, typically this will be in your home fiat currency.
Cost of electricity: All mining takes power to run the equipment, the more power efficient the mining process, the less power used and the more profit is available, typically this will be in your home fiat currency as well.
Price of the crypto-currency: This is where it gets more variable, your home fiat currency will constantly fluctuate against Bitcoin, and the cryptocurrency you are minig will also constantly fluctuate agains Bitcoin; this means its hard to know what profit you are likely to make on a weekly basis. However if you spend some time in the markets, you will notice that currencies go through cycles of high/low prices, the wise miner holds onto his/her mined crypto-currency until the next high price cycle, before selling, to maximise profit.

Yes you can mine Gridcoin on those old macbooks, though it probably wont make you much money at all. Yes Yes, if you have free power as part of your rent this will maximise any profit, and yes if you have enough solar power you can mine for free, at least in the daytime. The great thing about Gridcoin apart from being able to use old/unused kit, is you will be doing real scientific research, so even if your profit is insignificant, you will have done something good for humanity.
I would advise you to start small with kit you already have, and as you learn you can then choose to buy more powerful equipment.