Frankly, the rental market conditions in your area (i.e. where you buy) are extremely important and also their long term stability. If you are in Knightsbridge then you are pretty well assured fairly strong demand almost indefinitely. At the opposite end of the scale that aspect can be a rather dire prospect. Wherever you buy you need to decide whether to charge at the top of what the market will tolerate (covering all outlays but more difficult to fill/sell) or below average (vulnerable to knocks such as arrears, damage, onerous new legislation - but the best guarantee for bigger choice of tenants and long stays). Being on the average gives you average result: constant hassle at the average rate.
We have been letting a property for more than 35 years and on the whole we have been lucky, it has suited us until more recently. We have taken the latter approach above (below average rate of rent). The flat is part of the building we live in and having control of our surroundings has always been key (no mutual maintenance reluctance). But we find that even though we try to treat our tenants well, even become friends with them, it is in the end disappointing to then often find that they actually harbour the "us and them" attitude and when leaving they suddenly assume we have been exploiting them and all manner of defaults, damage, etc. suddenly crop up. Renting is one form of investment which provides an income but there are others and I would now in many ways like to be rid of the whole thing including our home.
I am assuming you are in the UK, including all your assets. From our experience until now and if I were you, I would move any surplus cash assets out of the UK and into a good foreign currency, the Swiss Franc (CHF) probably being the most impressive historically. Attachment to where we live (house and area) was what led to our buying the other unit, not the attraction of the investment as such although we always hoped not to actually lose on it. If our situation changes in the right way, we will sell everything and radically change our entire strategy.
Obviously, you will in the end make your own decision. Think about how you would cope with repeated hassle over the next 10, 20, 30 years - even if you have an agent you will still get the end result through them: Bad tenants, no tenants for a time, overheads both in agent fees, damage and onerous regulation, etc. Jacking the rent up may solve some problems but that makes others even worse. Being a landlord is not a magic carpet any more than the classic dream of owning a pub. Nobody likes a landlord, us-and-them is a national disease in the UK. If you are considering buying to let for the coming 5-10 years only then don't.