Clarifications. First of all, the purge of 37-38 probably only accounted directly for about 700,000 deaths, the majority of whom were party members. Most Soviet citizens weren't party members. A more important event was the deliberately planned regional famine of 32-33, which is thought to have killed 6-7 million people, most of them Ukranians (the Ukranian peasantry were a particularly troublesome source of rebellion since the very beginning of the Soviet Union). The famine of 1922, during which Lenin
commanded certain rebellious regions to deliver implausibly high quotas of grain to the state, seems to have caused around 5 million deaths by itself.
On Stalin's Ukranian famine of 1932-33, I would reccomend Robert Conquest's Harvest of Sorrow. I don't know of a book dealing only with the famine of 1922, so I'll just suggest reading the section in The Black Book of Communism (very good book) which concerns this event. One of the most comprehensive books on Stalin's purges is The Great Terror, also by Robert Conquest.
I think the conclusion you'll come to is that relatively speaking, the terror gets an unjustifiably large amount of attention compared to the famines. My personal suspicion is that this is because the terror affected intellectuals and politically active people, whereas the famines were aimed at breaking the resistance of rebellious peasants. There is a similar unjustifiable tendency to emphasize China's cultural revolution over the Great Leap Forward, where the same comparison holds (the Great Leap killed far more people - perhaps 30 million - but it largely spared the intellectuals).
60 million is too high for all of Russia's crimes combined, probably by a factor of 2 or 3, unless we arbitrarily start throwing in deaths which occurred during wartime. Not that I find such death more acceptable - it's just that they're less unique to communist governments, which usually commit their worst atrocities during peacetime, against their own citizens.
If Stalin had been assassinated, the chances are that Russia would actually have been better defended against Hitler. It also would probably have been more industrialized. Analysis suggests that by the early 30's the average Russian was actually worse off than the average Russian living in 1917. Imagine the Great Depression, only three times deeper and much longer. And the first five year plan under Stalin sure as hell didn't improve the economy - it was such a magnificent failure that by its conclusion, many of Stalin's enemies believed that all they had to do was sit back and wait for Stalin to be destroyed in the political fallout. They must have been surprised when he had them all shot the next year. As for the country's military preparedness, it was absolutely devastated by the purges. During the purges, Stalin killed off/imprisoned about 90% of the officers as part of his general policy of eliminating anyone in power who might conceivably become disloyal to him. If you're curious about the results of this, look into Russia's initial failure in its (unprovoked) attack on Finland. Or study the way the Nazis beat them crap out of them when they broke the non-agression pact.