Many of us do a minimalistic build for varying reasons. I have a medium size bus and my actual investment is about $6k so far. I got a good deal on this bus for about $2,500, put a set of new tires on it for $2,600, stripped out the interior and put in foam insulation for $800. I'm sure I've got about five hundred dollars worth of plywood and rigid insulation for my interior. Soon I'm going for my $100 paint job if it quits raining for long enough.
How much you spend on nice stuff for your bus is obviously up to you. There are a significant amount of free and cheap items that can be used in a bus. It is possible to use regular furniture in your bus but you'll get tired of strapping everything down each time you want to drive anywhere. So I'd say yeah you can build a bus for $10k or less. Most of us minimalists don't do the plumbing, using a composting toilet and/or public restrooms.
There are also varying reasons why some of us are minimalists. In my state I'm unable to insure a self built bus to motorhome conversion. I can however insure my bus as a "van". So I can do all the things people do with a regular size van. I just have to avoid the things that would qualify my van as a motorhome, like plumbing, mounted propane tanks, mounted water and sewer, electrical system, etc.
Some people say that us minimalists are just camping in a bus. I'd rather camp in a bus than to camp in a tent anyday. I may be the strange one because I'm boondocking in my own driveway on the grid with cable and internet. I live in the mountains anyway so I've got a great view out of all the windows.
You'll get lots of ideas here that can help you make an inexpensive build.
Welcome and good luck.