This is a great option to keep your housing costs down.
Keep in mind that housing costs more than just the mortgage, here are my personal rules of thumb:
1. Repairs and up keep run about 1-1.25% of the cost of the building each year. I am handy and do a lot myself, so budget slightly more if you sub everything out.
2. Insurance runs about $50/month per unit. It can get as low as $35/month per unit with larger volumes of units at one location. It can also be slightly higher if you carry more coverage than I do.
3. If the building has any shared utilities, factor those costs in.
4. Vacancy rates are extremely tough to account for. My area (Bay Area) is between 1-2% vacancy right now, I use 5% to be safe. This is an area you shouldn't underestimate because vacancies can be lengthy. To calculate a monthly loss due to vacancy, reduce the rent by the vacancy percentage (example $1,000 rent would net you $950 after accounting for a 5% vacancy factor).
5. Property taxes, don't forget those. They run approximately 1.3% the cost of the building (in my area) each year. They vary widely, so understand property tax in your area.
6. Lastly, consider including landscaping as a service for your tenants. It is unlikely that tenants will do any landscaping work for their own unit, this service can run about $200/property per month.
I have lived amongst my rental units for many years now and it is great. It takes very little time to attend to an issue at any property because they are so close. Some people think they wouldn't want to live near their tenants, but if it is a property I wouldn't want to live at then I don't buy it. I select tenants that are financially stable, but also someone I could live next to as a person. It's very doable and can keep HCOL areas affordable.