That will work and provide power to the wagon however it is not recommended for many reasons.
It is not recommend using a car starter battery in a series wiring arrangement with sealed lead acid batteries because they are bound to be different sizes so the larger battery will deplete faster than the smaller batteries do which could cause damage to the smaller batteries from being over-discharged. Also mixing different brands and sizes of lead acid batteries together into a single battery pack is not recommended because the different chemistries and electrical ratings of the batteries may not be compatible with each other which could cause the batteries to wear out faster than normal.
Another thing to consider is that car starter batteries are not designed for deep cycle use such as they will encounter when used on an electric wagon. They are designed for short bursts of high power to start an engine, and then to be immediately recharged. Starter batteries have thick lead plates which can output high amounts of current for a short period of time, as opposed to deep cycle batteries which have thin lead plates which can output low amounts of current for a prolonged period of time. If a starter battery is used for deep cycle conditions then it will not perform very well and will not last very long.
None the less if you have a couple of used starter batteries laying around that are not being used then there is no harm in making a 24 Volt battery pack out of them and seeing how long they will power the wagon for.
I want to build a reasonable electric wagon or wheel barrow that can handle weight and travel faster than walking speed as I am a general contractor most everything available is gas powered and expensive. I have a large retaining wall project involving hauling block and gravel would appreciate any kit recommendations
We can run some calculations to determine what size motor will be needed. I need to ask some questions for the calculations.
Does the wagon or wheelbarrow need to be able to climb inclines, and if so then what percent of grade do you want it to be able to climb?
How much weight will the wagon or wheelbarrow need to be able to carry?
What is the top speed in MPH that you want the wagon or wheelbarrow to travel at?
As I will be using this for all types of materials,dirt,gravel,concrete blocks it should be able to carry fully loaded 200 lbs .As to the desired speed most others only travel loaded at 2.6 -1 mile per hour but that is too slow for job situations if it can go at least 5 miles per hour that would be better.In most situations my work would involve flat grounds but it would be nice if it could handle a 15 percent grade.Also most times I would be using this for intermediate work meaning load some materials ,haul this to a back yard and back for multiple trips then it would set for afew hours and then be put back into work
Hope this helps