Hi @Verity Macdonald and welcome to real estate investing as one of the best ways to financial freedom. Here are my thoughts on your post. While I am also new to the area, this has worked for me in Boston (which is also a big city, so I am being generic)
CENTRAL OR SUBURBS?
The ROI in my experience is always higher in suburbs as long as they're close enough to public transportation and the city. For example, in Barcelona, that would be areas like Rubi or Terrassa. You'll put less money down and get a similar or higher ROI than a property in central.
ONE OR MORE?
It really depends on the actual numbers but I am a big fan of diversifying as it reduces risk. For example, one of my units right now is vacant and my ex-tenants left a big mess behind them. This unit will be undergoing repair for at least 2-3 weeks. That's 2-3 weeks of lost revenue. The other units being rented definitely make up the lost revenue.
MORTGAGE OR NOT?
Mortgage 100%. The rates are very low right now here. Try to put down the minimum possible down payment and have the tenants pay your mortgage. You're not paying for it so while you're missing on that additional profit, think of it as money set aside for more investments. Also, we're due for a correction so don't you want to save some of that money that you have to invest later when the prices are lower?
RENOVATIONS
This is tied to the quality of the tenants you're trying to bring in but keep in mind that the more you renovate towards luxury you're hurting your profits, adding maintenance costs, and relying on tenants to maintain the value (which they won't). Try to find that balance between quality and necessity.
HOUSE-HACKING?
I am a big fan of house hacking. Did it for 2+ years and saved so much money (think of this as profit, as it's money saved). @Taylor L. said it perfectly and I agree with him.
Hope this is helpful!
