It would be a lot easier to fit a good used diff. I do a few 1 Series and E90 diffs and they're not easy.
On an E36 diff though; Diff out, driveshaft flanges out (they clip in), unbolt the rear cover (catch the oil) and unbolt the side plates that hold the side bearings in. The diff will then lift out. You'll then pick the pinion nut nut locking plate out and undo the big pinion nut. The prop flange then pulls off, often with a bit of BFI if it's stuck. Refit the pinion nut and clout the pinion down until it's loose. Then pick the oil seal out, undo the nut again and remove the pinion. That's the easy bit.
Then you have to drive the outer bearings from the diff housing. The end one isn't too bad, the big inner one can be a right b*****d. There can be a spacer shim underneath. Now you have to get old bearing inner race from the pinion. That involves careful use of a grinder to narrow down and split it. You ideally need a press to fit the bearings and a selection off shims to set the pinion/crownwheel backlash up plus you really need to know what you're doing to set the 'crush' tube between the two pinion bearings.
Unless you've done these before, I'd avoid the whole thing. Good E36 Compact Z3 1.9 diffs are for peanuts. Or if yours is a bit whiney, drain the oil and refill with EP140 - that quietens them down a bit.
