The apartment was great, it has everything that you might need during your stay, it is situated in really fantastic district. In my opinion St. Elena is the best part of Venice to see how local people actually live, it is calm, incredibly clean and beautiful!
Luca was a great host before and after our accommodation. The last day it was raining and he came to bring us umbrellas! I want to really thank him for everything.
As for distance and transport- the apartment is very easy to find, it is 5 min walk from the Actv stop "St. Elena" ( 20 minutes with line 5.1 or 6 from P. Roma and 1 hour with line 1, which goes trough Grand Canal and is great to have some pics), and 15 min from St. Marco if you follow the alley along the seaside.
So, definitely a good choice : )