You can usually find old full suspension bikes on craigslist for less than $300. A lot of the older ones have substantial space in the main triangle. I have a '96 Specialized FSR Ground Control which has the same triangle space as a road bike. 2.5" rear travel.
Check this thread:
https://endless-sphere.com/forums/vi...2056&hilit=fsr
The EM3EV frame is pretty cool though.