Hi,
From my experience with the 1000W motor it probably will get you to 25 mph, however you'll find that the exceleration will be poor without the use of pedals, especially with four heavy lead acid batteries.
My advise is not to bother will SLA batteries because:
They have a much short life span so are only superficially cheaper
They will offer you less range then a comparable Lifepo4 battery due to their weight
They will have less ecelleration compared to Lifepo4
They suffer from voltage sag