Thoughts on how to proceed

[Stormwater]

So I picked up this Japanese maple cheap. Possible to reduce? How low would
You cut?

 

[Shibui]

No problem reducing that Japanese maple. You have several low shoots and it's almost always safe to cut back to a live shoot. JM can also be cut back to bare wood but there's a small risk they won't activate new buds or may die back. Very little risk where you have a healthy side branch.

How low would
You cut?

Depends entirely on what aims I had for the tree.
Chop to the 3rd branch for a moderate sized, taller bonsai. Still leaves a bit of straight trunk with no taper but also safest option.
Chop to 2nd branch for better taper and lower bends in the new trunk. That branch appears strong enough to maintain sap flow in the entire trunks and all roots but a slight chance (as always) of some dieback on the opposite side of the trunk.
Chop back to the lowest branch is more risky but has greatest potential to produce a shohin sized bonsai.

I would probably investigate the roots to find out what potential nebari is there before making a final decision. It's pointless developing a good trunk and branches if the roots don't look right under it.

 

[AmellLeo]

So I picked up this Japanese maple cheap. Possible to reduce? How low would
You cut?

Chopping it down to the second or third branch seems like the safest move. Might be worth checking the roots first and best to prune in early spring or late fall to keep it Healthy.