No you don't. If you want to invest in bitcoin all you need is to have income that is greater than your expenses. Now if you want to continue to buy beyond the first purchase and/or you want to hang onto your bitcoin and not end up spending it to cover your future expenses, then you have to have enough income or savings to be able to pay for your future expenses.
Keep in mind there is one point in time in which anyone might buy bitcoin and expect to be able to continue to hold the bitcoin and to buy more bitcoin in the future, yet the future is not completely known. Each of us might strive to keep our job or we might strive to control our expenses and/or to otherwise manage our cashflow with back up funds, but if we end up having future expenses that are greater than our income and we deplete our various forms of savings and/or back up funds, we may well end up having to spend our bitcoin at a time that is not completely of our own choosing.
You’re absolutely right, There is every tendency that if we are able to have an income that is greater than our expenses we should have a left over money which we can always refer as our discretionary income which is the best possible means of investing into Bitcoin, because we wouldn’t want a situation were we don’t have enough money to pay our bill and for our expenses, then there is also every tendency that we might not have a means of investing into Bitcoin, because if we end up using money meant for our expenses to invest in Bitcoin, I must say that is a bad bitcoin investment approach and we might still end up using our Bitcoin to pay for our expenses, that is why it’s more ideal to create discretionary income for Bitcoin investment, so we would wants to have other plans in place should our expenses increase while we are still in our accumulation phase.
Perhaps we would always try to create other opportunities for financial enablement, and also trying to have different types of cash flow as time passes, because there is a tendency that our expenses might increase as time goes along, what can we do, we might not be able to deal with our present income and we would wants to devise other means of generating revenue for paying our expenses so we probably wouldn’t have to fall back to our bitcoin investment, we might also want to exercise our savings and reserved funds and also have to generate other streams of income to be able to sustain our financial enablement while we also try to buy and accumulate more bitcoin which is a primary goal of everyone who is buying and holding bitcoin for a long term purpose, if we eventually have this process in mind we might end up avoiding whatever that will push us to fallback to our Bitcoin investment, instead we would eventually increase our discretionary income to buy and accumulate more bitcoin into our portfolio.