Nope, you got that mixed up. Series wound is the most common motor type for DIYers.most projects that I have seem use DC shunted motors
Expensive.Why AC is not the preferred type?
Nope, got that one mixed up too. An AC-MOTOR has higher efficiency than a series wound DC-motor but an AC-CONTROLLER has lower. In the end the difference is generally to the AC systems advantage, but the difference is also generally speaking pretty small when you consider what you get for your money. Buying more batteries to compensate is usually more economical.I understand that AC has cero mantenaince but smaller efficiency
For more details I recommend the search function. This topic has been convered several times in this forum in more or less friendly tone...