For property maintenance, you can use this checklist:
https://www.goodhousekeeping.com/home/a35875791/home-maintenance-checklist/
https://hoa.org.uk/advice/guides-for-homeowners/i-am-managing-2/home-maintenance-checklist/
https://www.swinton.co.uk/spotlight/your-home/maintenance-checklist/
Property maintenance is essential for both flats and houses. Maintaining a flat is much more challenging because you do not have control over the roof, other apartments and the external wall.
If there is a leak, you must first pay a plumber to investigate, pass the result to the management company, and argue with the flat owner upstairs. The whole process can take months.
It is much easier and quicker to fix a leak in a house. You hire a plumber and carry out the work, all fixed within hours.
Avoid listed buildings. There are lots of restrictions on what you can do. They cost more to maintain. Leakier and less insulated.
Yes, there are still lots of potential for freehold property development in north London. There is a significant shortage of properties in London, and freehold houses with gardens are highly sort after. After Covid lockdown, people are placing more value on outdoor space and an extra bedroom.
Garden - as long as there is no Japanese knotweed, it is not a problem. It costs a few grand to fix at most if there is no protected tree, and you don't want to chop any trees down.
If it is a London medium size overgrown garden with no protected tree and no tree to chop, excluding rubbish disposal, takes 2 men a few hours to clear out completely. I organise gardening services for my tenants.
You can find a gardener on checkatrade:
https://www.checkatrade.com/Search/Gardener/in/London
You can find a cheaper gardener if you want to clear stuff out. Then hire a landscape gardener separately.
To give you an idea of the costs:
https://www.checkatrade.com/blog/
If you are worried about the garden, I think you should hire an architect also to project manage the refurbishment. Get multiple quotes and ask to see their finished work. Do not pay a lump sum; pay in stages depending on their work.
https://www.homebuilding.co.uk/advice/how-to-choose-an-architect
https://www.houzz.co.uk/magazine/how-to-choose-an-architect-stsetivw-vs~159203159
https://hoa.org.uk/advice/guides-for-homeowners/i-am-improving/how-do-i-find-an-architect/
You can also ask RIBA to refer you to an architect:
https://www.architecture.com/working-with-an-architect