I’d buy the shitty flat.

You’re 50. You don’t know how long you’ll be able to work to pay off the expensive flat, nor do you know what needs you’ll have as you get older. You can buy the shitty flat, pay it off quickly, and you’ll own it. If you lose your job, you won’t be evicted for not paying rent. Depending on where you live, you’ll still have condo fees, property taxes, or whatever, but you’ll still have more leverage and leeway than simply being a tenant.

Don’t just grab any shitty flat; think of what you want now, and what you’ll need as you age. You may not still be in the same flat 10 or 20 years from now, but make that assumption and plan with that in mind. I don’t know what you want now (lots of kids vs no kids around, etc), but some things to think of as you age:

• People hanging around outside: this may be difficult to notice this time of year (I’m assuming you’re in the northern hemisphere), but kids playing, a community garden, someone sitting on their porch smoking - anything that speaks to a level of comfort being outside. That generally means more neighborhood spirit / community involvement, and generally means less tendency toward future crime.
• Some neighbors younger than you, and at least a few kids. When you get older, you’ll occasionally need help, and asking younger people you know is the easiest way.
• A grocery store within reasonable distances that is accessible either by walking or public transit.
• Minimal stairs to get into the building, minimal stairs to get into the flat, and minimal stairs within the flat. Stairs become more of a barrier and more of a danger as you age.

Probably other stuff as well, those are just the first things I thought of.