After your first year (getting in, rented and set up) aim for expected costs to be around 60-70% income. This covers you for vacancy, finders fees, upgrades and repairs. We bought our place to rent to our eldest daughter and her boyfriend so we went in expecting to lose money. They stayed less than a year. We then spent a E36 M3 load on maintenance, repair and upgrades. We could have advertised on Craigslist but decided to go through a realtor to get it rented. Although we had to pay a finder’s fee it saved a lot of hassle on screening people, getting references etc. and was worth it to us.
Also look for hidden fees. Where we rent there is a bi-annual city inspection of all rental properties to make sure they are in good condition, safe and up to code. Where we are it's $50, just a little to the South in another neighborhood it's $250. Things like that add up fast.
Also think carefully of the neighborhood. We looked at several and chose an up and coming area. We could have got more for less close by, bet we don't know how those neighborhoods are going to developed. We are safe in the bet that even if there's another downturn the neighborhood will ride it, it's a hot market. other places not so much.
We don't use a management company as we're only 20 mins away. We have friends who although more than capable of building a house from scratch themselves have 5 properties 2 hours away. For that reason it's worth the management fee so as not to have to shoot half way across the state at the drop of a hat.
Also most rental contracts say that things light bulb replacement, dripping faucet, minor issues are the renters responsibility be prepared for calls to fix all those simple things, especially if they are young and even more so if a single mother. We have other friends with several properties who rent to section 8 people. This is good in that you get guaranteed rent from the government, but bad in that you will be called for the most insignificant thing, many of these people can't even use a plunger on a blocked toilet.
One more thing. GET GOOD INSURANCE and possibly more important make sure the renters have their own INSURANCE, get a copy of the contract. We've been lucky, others haven’t with places being trashed. IF they cancel the contract after you have a copy you at least have something to go after them with in court if you need too for fraud and breach of contract. Don't be afraid to start eviction if people are late paying. Some friends do it straight away, others have tried to play long, be fair and work with tenants, guess what? They are still trying to play catch up with the renters 18 months later.
Don't be scared. We are not looking at this as immediate income. We are looking at this as guaranteed income in 15 years when it's paid off. It's part of our retirement, not our current living finances.