The 70mm back will be fine on your ELX, as will any of the 120/220 backs.
For lenses, the only brand choice you have is Hasselblad (optics made by Zeiss) . There are (were) some aftermarket teleconverters, but no other company that I know of made compatible lenses.
The classic standard lens is a 80mm f/2.8. As with most of the other components, there are new and old styles, along with some variants along the way. The oldest are the "C" series lenses, newer are CFI and CFE, which have different shutters, and controls that are a bit more convenient, and are multi-coated. Mostly, old and new are optically the same, but there are exceptions.
If you consistently prefer a wider or narrower view, you may want to consider a 50 or 60mm lens as your "normal" on the wide sider, or the 100mm lens for the tele side. The 60 is more or less equivalent to a 35-40mm lens on a 35mm camera, while the 50 is more like a 28. On the tele side, the 100 is akin to an 80'ish mm lens for 35. For tight portraits the 150 is very popular.
New replacement parts are no longer available for the C lenses, so you may want to look to the newer series for your lens choices. But, you'll spend a bit less on C series lenses.
For viewfinders, many of us prefer the 45 degree prism finders. The newer PME and PM5 prism finders are a bit lighter than the older NC-x units but either would work. For waist level finders, there are two styles, on the oldest one you have to fold down each side and then the back then front when collapsing it. The newer one has hinged sides and is a little quicker and more convenient to collapse. I like the newer one better, but in use they work the same. I mostly use a waist level finder on my ELX because of weight, but I use a prism on my manual body.
Much depends on your taste and style of working as to what sort of pieces to complete your kit, I've only hit the high points of your choices.
For manuals and other information, Hasselblad Historical is a good source,
http://hasselbladhistorical.eu/HW/HWVSys.aspx
And, btw, welcome to the Hasselblad end of the pool, come on in, the water's fine.
