Its not easy.
An Xpan is not the kind of camera you're likely to get can't be arsed about, but it'll cost you 2k to find out.
Xpan-esque ratio is 3-1 ish, so if you're looking at shooting 35 in a 120 camera, its gonna have to be 6x7, and to minimize distortion (if that matters to you) 60-65mm lens.
6x8 is borderline and 6x9 is just too long unless you fancy throwing half your film away.
So that narrows your options down to next to nothing.
You could shoot with a standard lens but they're mostly 90.
Nearest option is a fuji gsw680 65mm lens, the older 6x7 doesn't come with a WA, the newer has a WA option but cost is in xpan country.
120 pano is 617.
Obviously I'm only covering the cheaper RF's, there are other more expensive 6x7 RF's like Mamiya 7, Plaubell Makina but we're getting in to xpan country again.
Not covering 6x7 box and SLR's either since the Xpan is an RF, but you could shoot 3-1 on an RB or Pentax 67, but they are big lumps comparatively.
There are other 6x7 oddballs like linhof 220 and horseman convertible, but they're not that cheap and difficult to find.
If you're not bothered about 3-1 or are prepared to crop horizontally, any 6x6 will do, or in fact your 645...
