Complete noob stuck on inflate

Totally green here. Forgive and be patient…

I downloaded an stl file for 3D printing. The problem is that it was a 1:1 scale railway freight wagon, which is fine, but when I scaled it down to 1:148, the wall thickness also went that way.
Needless to say, there’s nothing there to print.

I know to use Sculpt > Inflate, but after that, I have no idea what to do. All the tutorials are for balloons and animation, which is not very helpful for a single “frame” which needs all its surfaces thickened to about a millimetre.

Has someone used this tool for a similar problem?

My wild guess would be - Sculpt is not the tool for this.
I’d rather get rid of the thickness of the model by selecting outer faces, hide them and delete what’s left or use tedious manual cleaning.
In edit mode select all and Delete - Limited dissolve could help get rid of most of the triangles which would make selections easier.
Then scale, apply scale and try to use Solidify modifier to get needed thickness.