To answer your specific question "Do I need to buy a warehouse THIS turn, for 1-2 turns down the road?" here are my informal steps for evaluating.
How many and how many types of goods are you producing? How many and how many types of goods are being produced by others? How many Warehouses are left? What is the opportunity cost to buying the Warehouse?
If you are producing few goods and think you will be able to ship and hold onto what doesn't ship in the next round you can hold off. You can begin planning for a Warehouse purchase and minimizing the hit to your doubloons (i.e., pick up a Quarry, select Builder, Trade what you fear might not ship).
If everyone (including yourself) is producing only enough different types of goods to fill the available ships you need to look at quantities and decide if you will be able to fit your goods onto one of the ships. If you are producing a different good than everyone else and can get it on a ship so that you have guaranteed space for a round or two you don't need a Warehouse immediately.
Small Warehouses are a scarce resource, in the sense that there are not enough for everyone to get one. Large Warehouses cost significant Doubloons and in games I have played I don't ever recall them all being built. Do you think other players will build the Small Warehouses so that you will not be able to? If so build now. Also, there is an opportunity cost in terms of denying or not denying another player the opportunity to build a Warehouse and store their goods.
I have noticed (and we've talked about it in my PR group with mild consensus) that it is easier to make a decision to buy a Warehouse earlier in the game than later. It seems tied to the opportunity cost. Early in the game choices are limited by what can be afforded so it seems less a sacrifice to buy the Small Warehouse. Later in the game it seems as if that higher priced building is more desirable because I've been working toward it so putting off building it to build a Small Warehouse is tough. I tend to try to buy a Small Warehouse early in the game. Sometimes I never need it, sometimes it saves me a lot of otherwise lost goods. The upside of the situation where it is built but never needed is that you get the points at the end of the game. If you build City Hall the Warehouses are violet buildings and will add points that way too.