I agree with those above saying that the best option is to build an essentially defensive Baltic fleet. Make the main priority of the fleet to defend the eastern part of the Gulf of Finland. This includes capability to quickly and extensively mine the waters west of Kronstadt. Superheavy coastal artillery can be used to protect these minefields, and you could also build some monitors to extend gun ranges against enemy ships that venture towards Leningrad. MTBs are also a cost-effective component for this kind of a system, and an argument could be made for developing (land-based) naval aviation, too.
However, if your doctrine includes any offensive components, you'll probably need at least submarines and possibly destroyers and destroyer leaders. If you plan to take Estonia and Finland, and the Estonian islands and the Ålands to extend the Leningrad-area defence to cover the same area as during the Tsarist times, you'll need ships to overcome the Estonian and Finnish naval defence. Of these two, only Finland has significant surface combatants in the Väinämöinen and Ilmarinen. To counter them, you'd conceivably need at least some ships with at minimum 10-12 inch guns, whatever form these ships take (monitor, coastal defence ship). If and when you then take Finland and Estonia, with those islands, these vessels will be useful for protecting your new coasts agains enemy operations. Bases in Finland and Estonia can also be handily used to try to strangle Swedish iron ore transports to Germany with submarine warfare.