The flat is great and the host is even better!
We stayed at Kostas place for one week. The flat is spacious, clean and well furnished. You have two balconies and a nice kitchen with a fridge.
It is located a little off-center, but you can reach easily by foot or by bus that literally leaves literally on the other side of the road.
We discovered Thessaloniki completely by foot - no problem as well.
There is a pizza place on the next block, at least two supermarkets less than 5 minutes walking distance away and many bakeries and small shops close by.
Kostas shared many tips on what to do in Thessaloniki with us. He is a very heartwarming person and even stopped by for a little chat as he was passing by on his bike while we were at the beach. He even have us a lift with his car on the first evening to a nice viewpoint over the city.
Special tip: If you want to go to the beaches, don't take the bus. They are usually very crowded and not much fun to go with, in Thessaloniki. Instead use the ferry that leaves at the pier and also stops at the white tower. It's 3.50€ per trip but more than worth the money.
Also, if choosing the ferry, get off at the second beach, not the first one, as it is way bigger while also being less crowded by tourists.