If you want it level, cut furring strips and put up drywall.
If the precise shape of the ceiling is going to bother you, that might be worth the agony and is a heck of a lot easier than trying to get that much mud onto the ceiling, much less on the ceiling and actually level. I wouldn't bother with it, personally. The slight angle would not offend my sensibilities, and is also better acoustically than a dead-flat ceiling.
For smooth, drywall compound or actual plaster will do. I'd lean towards plaster for concrete, but I'm not sure there's anything to my bias, there, other than my bias. Grab a big ol' knife (or trowel) and go. If using mud, probably the 90-minute setting (aka "hot mix") compound, unless someone else has a better reason than my bias in support of my bias.
Then again, USG (and probably others) has a compound specifically for concrete.
COVER COAT® Compound is a premium mixture ideally used for filling and
smoothing above-grade, monolithic interior concrete ceilings and
columns, embedding paper drywall tape or acting as the first fill coat
on bead, trim and fasteners. The compound dries white for a quick,
easy finish.
Further reading:
Gypsum Plaster over concrete and Portland cement "plaster" (stucco) over concrete
There is visual evidence in photo A3 of one or more interior renovations constructed with less than workman-like quality.
Given the age of the house and what portions of the original construction remain, I it would be common for a house of this age to have had a load bearing wall perpendicular to the ceiling joists when originally constructed.
If there was previously an interior load bearing wall perpendicular to the ceiling joists, it's purpose would have been to reduce ceiling joist deflection. The cracking in the ceiling finish is symptomatic of deflection beyond the strength of the finish joints.
I recommend consulting a structural engineer familiar with residential projects of this scope. The condition is probably repairable given a reasonable budget, but it is necessary to first know what exactly needs to be repair.
Best Answer
There are many approaches. One of the least labor-intensive (but folks who have not gone down the route of removing wallpaper, etc. almost never believe it) is to apply a new layer of drywall (thinnest you can source) over the walls & ceiling you don't like the paper/texture on. That spares you from hours of steaming wall paper and fighting with the resulting wall (usually not) taking paint nicely, and filling and/or scraping off the ceiling texture. You may need to extend electrical boxes, but you get a nice new paintable surface without the fuss of removing the old one.
Or, if electrical or insulation upgrades would be good anyway, rip out the existing drywall and replace it.
Of course, if you "saw a house" the easiest approach is to not buy that one. If you buy it anyway, letting it be for a year to see if you still hate it enough to bother is often worthwhile.