The size engine is less important than its build quality. Whether a 1915, 2110 or some different size, get an engine built by someone you know is reputable. Get one with balanced internals, full-flowed oiling, and known, named parts. If the person selling an engine shrugs when you ask what cam and heads and valves are in the motor, pass.
There are still good VW engine builders all over this great nation, and not all of them are known to the dozen or so guys who post regularly on this board. But we do know of several no-great engine builders, GEX being at the top of that list.
If you find a used Speedster you like with an GEX engine or even one the owner can't tell you much about, budget $5k to go through and/or replace it with a better one. A stock old Bug engine will get you down the road reliably (if not with the gusto you'd like), but a "performance build" done poorly will usually fail catastrophically in short order.