I have (and use, although not much lately - grad school, arrgh) a bunch of 120 box cameras - Zeiss Box Tengor, Ansco Shur-Flash, a few others. I think they offer a lot more bang for the buck than a Holga. Plus, you can keep better eye contact with your subject using the top viewfinder - it's a small thing but I find it helps. I have found Tengors for less than $20 on ebay; the Anscos are probably more Holga-esque and can be had cheaper than that.
It's not a box camera per se, but another good "Holga-only-not" candidate is the Pho-Tak Foldex 20, a 6x9 American folder from the 1950s with a meniscus lens that seems to be common as dirt. Aperture, shutter speed, and focus are all fixed; the only controls are the shutter release and a switch between "time" (B) and "inst" (about 1/50).
I have been using a Brownie Target Six-20. Yes, it is a pain to respool the 120 onto 620 spools, but the camera does a good job with the meniscus lens. I took the front plate off the camera ( it is held with four screws) and cleaned the lens and view finders, also dusted out the shutter. My first camera was an Ansco ShurFlash. I don't know what happened to it, that is over fifty years ago. It used 120 film. Good luck in your search.
It may be far too sophisticated for you, but I suggest you consider a Type 1 Rolleicord. I have an Art Deco version, and it is a delight to use. The lens [if shaded from the sun] is amazing. They go for less than $100 on eBay. The Art Deco version is especially easy to work on - no leatherette to remove to pull the whole front off the camera. It may need a new mirror, which can be had for $70 + postage from Bill Maxwell.
With a Maxwell screen: remarkably bright for focusing. If you end up with one that has a tarnished mirror [very likely] you could do without [a new mirror] by using the sports viewfinder and estimating focus. There is a good depth of field table on the Art Deco's back. I'm not sure how that is handled on the Rolleicord 1 Model 2.
Dug up a picture taken with my Shur-Shot. This is what happens when you walk around with a box camera, someone asks you what kind of camera that is, and they click the shutter by accident. Oh well, it turned out alright!
The Dacora Digna is a 6x6 120 camera with a very simple lens/shutter that is generally to be found for $20 or less. I've had one for a few years but have not yet run a roll through it to see just how bad it might be.