I have lived in Sandgate since 2013 (great place to live) and the best streets as a real estate investment are probably as follows:
* Any Avenues address - starts at 1st Avenue near town centre (Cliff Street right next to it) then goes up to 23rd Avenue in Brighton. These streets are closest to the waterfront so quite sought after. Only downside is they are also very flat so flood liable during severe weather events (have seen this happen). The closer you get to Sandgate CBD the better.
* Streets near Shorncliffe boundary - Shorncliffe is the end of the line and water on 3 sides so quite sought after area. Anywhere near Shorncliffe or in it are good addresses.
* Sandgate Hill - overlooks Dowse Lagoon and best street on hill is Alexandra Street. Many homes here enjoy great views to Brisbane City and some even have a glimpse of Moreton Bay.