Personally I wouldn't take any of the Holga cameras seriously which is not to say a good picture has never been taken on one but they are definitely "art cameras"
There are not many options in eye level medium format SLR's which you seem most interested in.
Pentax 6x7, 67 and 67II Huge beast of a camera the originals with out lens can be found for maybe £150 the latest 67II with a lens is looking more like £1000ish although you might get lucky (I paid £660 for one with the 45mm)
It was a pro system camera there is a full range of lenses and accessories with the AE Pentaprism you even get aperture priority and matrix meeting only real potential downside other than being huge is the 30th of a second flash sync although a couple of leaf shutter lenses were made.
MF shutters tend to be leaf shutters on the lens and typically offfer 1 second to 1/500second with flash sync at all speeds, things like the few Eye level SLR's have cloth focal plane shutters which due to their size tend to be rather slow on the flash sync front.
There was also the Exakta 66 that hadSchnieder lenses also huge and probably still expensive also rare.
East German
Pentacon 6 are relatively cheap the Exaka is a more refined West German version if you like there is also the
Kiev 66 if that is of interest then MW classic have a sensibly priced one on
this page.
For street photography range finders might be a better bet if you have the funds the Mamiya 6 and seven are universally rated as excellent and have several lenses they are also in the £100o bracket there are also a range of options from Fuji in the £350 and up range in various formats while as Andy has said there is lots of choices in the Folders from the 1930's to 1960's.
While the folders are quite compact and pretty good optically the results do not really compare with the later models aimed at the professional market but they can easily provide better quality than 35mm film.
For convenience you could look for things like the Super Ikonta IV which had built in coupled range finder and a built in but uncoupled meter these are probably £250ish plus if you don't need a range finder or meter Agfa Isolettes and many others are £10 territory.
Edit
Picking up the
Medium Format Manual from Amazon for a penny plus the £2.80 postage cost would be worthwhile