Buy 2 Let Strategy - Northern Line

Hello, I want to buy another buy to let on the northern line in 18 months with my partner. Northbound areas being considered are; finchley, colindale, burnt oak, totteridge, edgware, hendon etc. I’m not sure whether its better to buy a three bed flat is a lesser area, or a two bed flat in a high end area as strategy. Or whether overall a house is a better investment than a flat. Overall strategy is equity growth over time. Any buy to let investors on here, that have purchased on the northern line. Budget 500-600 max.

You may struggle to get a house with that budget, but thats what I would aim for. Freehold is still much better than leasehold.

Any particular reason why Northern line?