Since you like square and have a bit of spare cash my options would be,
Yashica MAT 124g if you don't mind the TLR way of shooting, has a great lens and a decent feature set. You can get older Yashica MAT's but they'll probably need a service by a professional, 124g's were made until the 1980's so it's easier to get a working camera, but you generally pay more for that but save in the long run.
Iskra is a russian medium format rangefinder folder and a copy of the Agfa Super Isolette, has pretty much everything you could want from a folder though it's a little harder to find.
Franka Solida IIIe is a cheaper option, has a nice Schneider f/2.9 lens and a decent Synchro Compur shutter and an uncoupled rangefinder and can take 40.5mm filters.
Pentacon Six SLR system, usually cheaper than Mamiya's and Pentax's and weighs less (about 1.7Kg with a metered prism head), you can get waist or eye level prism finders and has a decent range of lenses.
It's pretty basic but OK i guess, you'll probably want a better camera quite quickly after using it. Will need a light meter for exposure and a rangefinder accessory (Watameter ones are my favourite) to get the focus distance to set on the lens. It's a nice cheap way into medium format though.
Which model?
There are several. Though out of that range the Isolette III is probably the best since it has built in uncoupled rangefinder. The Isolette I is about as basic as the Nettar and not much different really. Isolette II is a slightly upgraded I. If you want one go for a II or III and find one with a Compur-Rapid or Synchro-Compur shutter as they'll give you a good range of speeds, maybe a Prontor-SV at a stretch, just avoid the Pronto or Vario shutters since they are too basic really.