Multi channel is nice if you want to watch eight plugs or injectors at once, but two will do. You can construct an 8 channel switch fast enough to keep up with a car's waveforms fairly cheaply.
I can't think of a scope that's so slow it won't keep up with a car's waveforms. Automotive events are glacial compared to consumer electronic ones.
Any oscilloscope has a sweep function. Possibly you mean 'delayed sweep'. Yes, that's good to have.
A decent scope is likely to have plugin modules that can add capabilities.
A simple loop of wire around a plug lead will pick up all the signal you need.
A storage scope is handy to trap transient events.